diff --git a/functions.php b/functions.php
index 8716c197..a7fe5c8c 100644
--- a/functions.php
+++ b/functions.php
@@ -1,13 +1,19 @@
 <?php
 
-// phpcs:ignore WordPress.NamingConventions.PrefixAllGlobals.NonPrefixedFunctionFound -- Renaming breaks Phar compat.
-function wp_export( $args = array() ) {
-	$defaults     = array(
+/**
+ * @param array{filters?: array<mixed>, format?: class-string<WP_Export_WXR_Formatter>, writer?: class-string<WP_Export_Returner>, writer_args?: mixed} $args
+ */
+function wp_export( $args = array() ) { // phpcs:ignore WordPress.NamingConventions.PrefixAllGlobals.NonPrefixedFunctionFound -- Renaming breaks Phar compat.
+	$defaults = array(
 		'filters'     => array(),
 		'format'      => 'WP_Export_WXR_Formatter',
 		'writer'      => 'WP_Export_Returner',
 		'writer_args' => null,
 	);
+
+	/**
+	 * @var array{filters: array<mixed>, format: class-string<WP_Export_WXR_Formatter>, writer: class-string<WP_Export_Returner>, writer_args: mixed} $args
+	 */
 	$args         = wp_parse_args( $args, $defaults );
 	$export_query = new WP_Export_Query( $args['filters'] );
 	$formatter    = new $args['format']( $export_query );
diff --git a/phpstan.neon.dist b/phpstan.neon.dist
new file mode 100644
index 00000000..25183173
--- /dev/null
+++ b/phpstan.neon.dist
@@ -0,0 +1,16 @@
+parameters:
+  level: 9
+  paths:
+    - src
+    - export-command.php
+    - functions.php
+  scanDirectories:
+    - vendor/wp-cli/wp-cli/php
+  scanFiles:
+    - vendor/php-stubs/wordpress-stubs/wordpress-stubs.php
+  treatPhpDocTypesAsCertain: false
+  ignoreErrors:
+    - identifier: missingType.iterableValue
+    - identifier: missingType.property
+    - identifier: missingType.parameter
+    - identifier: missingType.return
diff --git a/src/Export_Command.php b/src/Export_Command.php
index 273f9d33..96ffbbb5 100644
--- a/src/Export_Command.php
+++ b/src/Export_Command.php
@@ -181,7 +181,7 @@ public function __invoke( $_, $assoc_args ) {
 			'wp_export_new_file',
 			static function ( $file_path ) {
 				WP_CLI::log( sprintf( 'Writing to file %s', $file_path ) );
-				Utils\wp_clear_object_cache();
+				Utils\wp_clear_object_cache(); // phpcs:ignore PHPCompatibility.FunctionUse.RemovedFunctions.wp_clear_object_cacheDeprecatedRemoved @phpstan-ignore-line
 			}
 		);
 
@@ -190,7 +190,7 @@ static function ( $file_path ) {
 				wp_export(
 					[
 						'filters'     => $this->export_args,
-						'writer'      => 'WP_Export_File_Writer',
+						'writer'      => WP_Export_File_Writer::class, // @phpstan-ignore argument.type
 						'writer_args' => 'php://output',
 					]
 				);
@@ -198,7 +198,7 @@ static function ( $file_path ) {
 				wp_export(
 					[
 						'filters'     => $this->export_args,
-						'writer'      => 'WP_Export_Split_Files_Writer',
+						'writer'      => WP_Export_Split_Files_Writer::class, // @phpstan-ignore argument.type
 						'writer_args' => [
 							'max_file_size'         => $this->max_file_size,
 							'destination_directory' => $this->wxr_path,
@@ -234,6 +234,7 @@ private function validate_args( $args ) {
 
 		foreach ( $args as $key => $value ) {
 			if ( is_callable( [ $this, 'check_' . $key ] ) ) {
+				/** @phpstan-ignore argument.type */
 				$result = call_user_func( [ $this, 'check_' . $key ], $value );
 				if ( false === $result ) {
 					$has_errors = true;
@@ -251,9 +252,14 @@ private function validate_args( $args ) {
 		}
 	}
 
+	/**
+	 * @param string $path
+	 *
+	 * @phpstan-ignore method.unused
+	 */
 	private function check_dir( $path ) {
 		if ( empty( $path ) ) {
-			$path = getcwd();
+			$path = (string) getcwd();
 		} elseif ( ! is_dir( $path ) ) {
 			WP_CLI::error( sprintf( "The directory '%s' does not exist.", $path ) );
 		} elseif ( ! is_writable( $path ) ) {
@@ -265,34 +271,49 @@ private function check_dir( $path ) {
 		return true;
 	}
 
+	/**
+	 * @param string $date
+	 *
+	 * @phpstan-ignore method.unused
+	 */
 	private function check_start_date( $date ) {
 		if ( null === $date ) {
 			return true;
 		}
 
 		$time = strtotime( $date );
-		if ( ! empty( $date ) && ! $time ) {
+		if ( ! empty( $date ) && false === $time ) {
 			WP_CLI::warning( sprintf( 'The start_date %s is invalid.', $date ) );
 			return false;
 		}
-		$this->export_args['start_date'] = date( 'Y-m-d', $time ); // phpcs:ignore WordPress.DateTime.RestrictedFunctions.date_date
+		$this->export_args['start_date'] = date( 'Y-m-d', (int) $time ); // phpcs:ignore WordPress.DateTime.RestrictedFunctions.date_date
 		return true;
 	}
 
+	/**
+	 * @param string $date
+	 *
+	 * @phpstan-ignore method.unused
+	 */
 	private function check_end_date( $date ) {
 		if ( null === $date ) {
 			return true;
 		}
 
 		$time = strtotime( $date );
-		if ( ! empty( $date ) && ! $time ) {
+		if ( ! empty( $date ) && false === $time ) {
 			WP_CLI::warning( sprintf( 'The end_date %s is invalid.', $date ) );
 			return false;
 		}
-		$this->export_args['end_date'] = date( 'Y-m-d', $time ); // phpcs:ignore WordPress.DateTime.RestrictedFunctions.date_date
+		$this->export_args['end_date'] = date( 'Y-m-d', (int) $time ); // phpcs:ignore WordPress.DateTime.RestrictedFunctions.date_date
 		return true;
 	}
 
+	/**
+	 * @param string $post_type
+	 *
+	 * @phpstan-ignore method.unused
+	 */
 	private function check_post_type( $post_type ) {
 		if ( null === $post_type || 'any' === $post_type ) {
 			return true;
@@ -317,6 +338,11 @@ private function check_post_type( $post_type ) {
 		return true;
 	}
 
+	/**
+	 * @param string $post_type
+	 *
+	 * @phpstan-ignore method.unused
+	 */
 	private function check_post_type__not_in( $post_type ) {
 		if ( null === $post_type ) {
 			return true;
@@ -341,6 +367,11 @@ private function check_post_type__not_in( $post_type ) {
 		return true;
 	}
 
+	/**
+	 * @param string $post__in
+	 *
+	 * @phpstan-ignore method.unused
+	 */
 	private function check_post__in( $post__in ) {
 		if ( null === $post__in ) {
 			return true;
@@ -357,6 +388,11 @@ private function check_post__in( $post__in ) {
 		return true;
 	}
 
+	/**
+	 * @param string $start_id
+	 *
+	 * @phpstan-ignore method.unused
+	 */
 	private function check_start_id( $start_id ) {
 		if ( null === $start_id ) {
 			return true;
@@ -374,14 +410,19 @@ private function check_start_id( $start_id ) {
 		return true;
 	}
 
+	/**
+	 * @param string $author
+	 *
+	 * @phpstan-ignore method.unused
+	 */
 	private function check_author( $author ) {
 		if ( null === $author ) {
 			return true;
 		}
 
 		// phpcs:ignore WordPress.WP.DeprecatedFunctions.get_users_of_blogFound -- Fallback.
-		$authors = function_exists( 'get_users' ) ? get_users() : get_users_of_blog();
-		if ( empty( $authors ) || is_wp_error( $authors ) ) {
+		$authors = function_exists( 'get_users' ) ? get_users() : get_users_of_blog(); // @phpstan-ignore-line
+		if ( empty( $authors ) ) {
 			WP_CLI::warning( 'Could not find any authors in this blog.' );
 			return false;
 		}
@@ -407,6 +448,9 @@ private function check_author( $author ) {
 		return true;
 	}
 
+	/**
+	 * @phpstan-ignore method.unused
+	 */
 	private function check_max_num_posts( $num ) {
 		if ( null !== $num && ( ! is_numeric( $num ) || $num <= 0 ) ) {
 			WP_CLI::warning( 'max_num_posts should be a positive integer.' );
@@ -418,6 +462,11 @@ private function check_max_num_posts( $num ) {
 		return true;
 	}
 
+	/**
+	 * @param string $category
+	 *
+	 * @phpstan-ignore method.unused
+	 */
 	private function check_category( $category ) {
 		if ( null === $category ) {
 			return true;
@@ -428,7 +477,7 @@ private function check_category( $category ) {
 		}
 
 		$term = category_exists( $category );
-		if ( empty( $term ) || is_wp_error( $term ) ) {
+		if ( empty( $term ) ) {
 			WP_CLI::warning( sprintf( 'Could not find a category matching %s.', $category ) );
 			return false;
 		}
@@ -436,13 +485,18 @@ private function check_category( $category ) {
 		return true;
 	}
 
+	/**
+	 * @param string $status
+	 *
+	 * @phpstan-ignore method.unused
+	 */
 	private function check_post_status( $status ) {
 		if ( null === $status ) {
 			return true;
 		}
 
 		$stati = get_post_stati();
-		if ( empty( $stati ) || is_wp_error( $stati ) ) {
+		if ( empty( $stati ) ) {
 			WP_CLI::warning( 'Could not find any post stati.' );
 			return false;
 		}
@@ -455,6 +509,11 @@ private function check_post_status( $status ) {
 		return true;
 	}
 
+	/**
+	 * @param string|null $skip
+	 *
+	 * @phpstan-ignore method.unused
+	 */
 	private function check_skip_comments( $skip ) {
 		if ( null === $skip ) {
 			return true;
@@ -468,6 +527,11 @@ private function check_skip_comments( $skip ) {
 		return true;
 	}
 
+	/**
+	 * @param string $size
+	 *
+	 * @phpstan-ignore method.unused
+	 */
 	private function check_max_file_size( $size ) {
 		if ( ! is_numeric( $size ) ) {
 			WP_CLI::warning( 'max_file_size should be numeric.' );
@@ -479,6 +543,11 @@ private function check_max_file_size( $size ) {
 		return true;
 	}
 
+	/**
+	 * @param string $once
+	 *
+	 * @phpstan-ignore method.unused
+	 */
 	private function check_include_once( $once ) {
 		if ( null === $once ) {
 			return true;
@@ -497,6 +566,11 @@ private function check_include_once( $once ) {
 		return true;
 	}
 
+	/**
+	 * @param string $allow_orphan_terms
+	 *
+	 * @phpstan-ignore method.unused
+	 */
 	private function check_allow_orphan_terms( $allow_orphan_terms ) {
 		if ( null === $allow_orphan_terms ) {
 			return true;
diff --git a/src/WP_Export_Oxymel.php b/src/WP_Export_Oxymel.php
index 0d3c75e3..39480a07 100644
--- a/src/WP_Export_Oxymel.php
+++ b/src/WP_Export_Oxymel.php
@@ -10,7 +10,7 @@ public function optional( $tag_name, $contents ) {
 
 	public function optional_cdata( $tag_name, $contents ) {
 		if ( $contents ) {
-			$this->$tag_name->contains->cdata( $contents )->end;
+			$this->$tag_name->contains->cdata( $contents )->end; // @phpstan-ignore-line
 		}
 		return $this;
 	}
diff --git a/src/WP_Export_Query.php b/src/WP_Export_Query.php
index 4ddc9656..02e46aac 100644
--- a/src/WP_Export_Query.php
+++ b/src/WP_Export_Query.php
@@ -122,7 +122,7 @@ public function custom_taxonomies_terms() {
 		}
 		$custom_taxonomies = get_taxonomies( [ '_builtin' => false ] );
 		// phpcs:ignore WordPress.WP.DeprecatedParameters.Get_termsParam2Found -- Deprecated, but we need to support older versions of WordPress.
-		$custom_terms = (array) get_terms( $custom_taxonomies, [ 'get' => 'all' ] );
+		$custom_terms = (array) get_terms( $custom_taxonomies, [ 'get' => 'all' ] ); // @phpstan-ignore-line
 		$custom_terms = $this->process_orphaned_terms( $custom_terms );
 		$custom_terms = self::topologically_sort_terms( $custom_terms );
 		return $custom_terms;
@@ -323,7 +323,7 @@ private function bloginfo_rss( $section ) {
 
 	private function find_user_from_any_object( $user ) {
 		if ( is_numeric( $user ) ) {
-			return get_user_by( 'id', $user );
+			return get_user_by( 'id', (int) $user );
 		} elseif ( is_string( $user ) ) {
 			return get_user_by( 'login', $user );
 		} elseif ( isset( $user->ID ) ) {
@@ -334,10 +334,10 @@ private function find_user_from_any_object( $user ) {
 
 	private function find_category_from_any_object( $category ) {
 		if ( is_numeric( $category ) ) {
-			return get_term( $category, 'category' );
+			return get_term( (int) $category, 'category' );
 		} elseif ( is_string( $category ) ) {
 			$term = term_exists( $category, 'category' );
-			return isset( $term['term_id'] ) ? get_term( $term['term_id'], 'category' ) : false;
+			return isset( $term['term_id'] ) ? get_term( (int) $term['term_id'], 'category' ) : false;
 		} elseif ( isset( $category->term_id ) ) {
 			return get_term( $category->term_id, 'category' );
 		}
diff --git a/src/WP_Export_Returner.php b/src/WP_Export_Returner.php
index 6327e135..234e1b5b 100644
--- a/src/WP_Export_Returner.php
+++ b/src/WP_Export_Returner.php
@@ -4,7 +4,6 @@ class WP_Export_Returner extends WP_Export_Base_Writer {
 	private $result = '';
 
 	public function export() {
-		$this->private = '';
 		try {
 			parent::export();
 		} catch ( WP_Export_Exception $e ) {
diff --git a/src/WP_Export_WXR_Formatter.php b/src/WP_Export_WXR_Formatter.php
index 3910cc8e..2f497396 100644
--- a/src/WP_Export_WXR_Formatter.php
+++ b/src/WP_Export_WXR_Formatter.php
@@ -12,11 +12,24 @@
  * Responsible for formatting the data in WP_Export_Query to WXR
  */
 class WP_Export_WXR_Formatter {
+	/**
+	 * @var WP_Export_Query
+	 */
+	private $export;
+
+	/**
+	 * @var string
+	 */
+	private $wxr_version;
+
 	public function __construct( $export ) {
 		$this->export      = $export;
 		$this->wxr_version = WXR_VERSION;
 	}
 
+	/**
+	 * @param array<string> $requested_sections
+	 */
 	public function before_posts( $requested_sections = [] ) {
 		$available_sections = [
 			'header',
@@ -72,7 +85,7 @@ public function header() {
 
 COMMENT;
 		return $oxymel
-			->xml
+			->xml // @phpstan-ignore-line
 			->comment( $comment )
 			->raw( $wp_generator_tag )
 			->open_rss(
@@ -92,7 +105,7 @@ public function header() {
 	public function site_metadata() {
 		$oxymel   = new Oxymel();
 		$metadata = $this->export->site_metadata();
-		return $oxymel
+		return $oxymel // @phpstan-ignore-line
 			->title( $metadata['name'] )
 			->link( $metadata['url'] )
 			->description( $metadata['description'] )
@@ -116,7 +129,7 @@ public function authors() {
 					->tag( 'wp:author_display_name' )->contains->cdata( $author->display_name )->end
 					->tag( 'wp:author_first_name' )->contains->cdata( $author->user_firstname )->end
 					->tag( 'wp:author_last_name' )->contains->cdata( $author->user_lastname )->end
-					->end;
+					->end; // @phpstan-ignore-line
 		}
 		return $oxymel->to_string();
 	}
@@ -132,7 +145,7 @@ public function categories() {
 				->tag( 'wp:category_parent', $category->parent_slug )
 				->optional_cdata( 'wp:cat_name', $category->name )
 				->optional_cdata( 'wp:category_description', $category->description )
-				->end;
+				->end; // @phpstan-ignore-line
 		}
 		return $oxymel->to_string();
 	}
@@ -146,7 +159,7 @@ public function tags() {
 				->tag( 'wp:tag_slug', $tag->slug )
 				->optional_cdata( 'wp:tag_name', $tag->name )
 				->optional_cdata( 'wp:tag_description', $tag->description )
-				->end;
+				->end; // @phpstan-ignore-line
 		}
 		return $oxymel->to_string();
 	}
@@ -173,10 +186,10 @@ public function post( $post ) {
 		$GLOBALS['post'] = $post;
 		setup_postdata( $post );
 
-		$oxymel->item->contains
+		$oxymel->item->contains // @phpstan-ignore-line
 			->tag( 'title' )->contains->cdata( apply_filters( 'the_title_export', $post->post_title ) )->end // phpcs:ignore WordPress.NamingConventions.PrefixAllGlobals.NonPrefixedHooknameFound -- WordPress native hook.
 			->link( esc_url( apply_filters( 'the_permalink_rss', get_permalink() ) ) ) // phpcs:ignore WordPress.NamingConventions.PrefixAllGlobals.NonPrefixedHooknameFound -- WordPress native hook.
-			->pubDate( mysql2date( 'D, d M Y H:i:s +0000', get_post_time( 'Y-m-d H:i:s', true ), false ) )
+			->pubDate( mysql2date( 'D, d M Y H:i:s +0000', (string) get_post_time( 'Y-m-d H:i:s', true ), false ) )
 			->tag( 'dc:creator', get_the_author_meta( 'login' ) )
 			->guid( esc_url( get_the_guid() ), [ 'isPermaLink' => 'false' ] )
 			->description( '' )
@@ -198,23 +211,23 @@ public function post( $post ) {
 			->tag( 'wp:is_sticky', $post->is_sticky )
 			->optional( 'wp:attachment_url', wp_get_attachment_url( $post->ID ) );
 		foreach ( $post->terms as $term ) {
-			$oxymel
+			$oxymel // @phpstan-ignore-line
 			->category(
 				[
 					'domain'   => $term->taxonomy,
 					'nicename' => $term->slug,
 				]
-			)->contains->cdata( $term->name )->end;
+			)->contains->cdata( $term->name )->end; // @phpstan-ignore-line
 		}
 		foreach ( $post->meta as $meta ) {
-			$oxymel
+			$oxymel // @phpstan-ignore-line
 			->tag( 'wp:postmeta' )->contains
 				->tag( 'wp:meta_key', $meta->meta_key )
 				->tag( 'wp:meta_value' )->contains->cdata( $meta->meta_value )->end
-				->end;
+				->end; // @phpstan-ignore-line
 		}
 		foreach ( $post->comments as $comment ) {
-			$oxymel
+			$oxymel // @phpstan-ignore-line
 			->tag( 'wp:comment' )->contains
 				->tag( 'wp:comment_id', $comment->comment_ID )
 				->tag( 'wp:comment_author' )->contains->cdata( $comment->comment_author )->end
@@ -229,16 +242,16 @@ public function post( $post ) {
 				->tag( 'wp:comment_parent', $comment->comment_parent )
 				->tag( 'wp:comment_user_id', $comment->user_id )
 				->oxymel( $this->comment_meta( $comment ) )
-				->end;
+				->end; // @phpstan-ignore-line
 		}
-		$oxymel
-			->end;
+		$oxymel // @phpstan-ignore-line
+			->end; // @phpstan-ignore-line
 		return $oxymel->to_string();
 	}
 
 	public function footer() {
 		$oxymel = new Oxymel();
-		return $oxymel->close_channel->close_rss->to_string();
+		return $oxymel->close_channel->close_rss->to_string(); // @phpstan-ignore-line
 	}
 
 	protected function terms( $terms ) {
@@ -253,10 +266,10 @@ protected function terms( $terms ) {
 				$oxymel
 				->tag( 'wp:term_parent', $term->parent_slug );
 			}
-				$oxymel
+				$oxymel // @phpstan-ignore-line
 				->optional_cdata( 'wp:term_name', $term->name )
 				->optional_cdata( 'wp:term_description', $term->description )
-				->end;
+				->end; // @phpstan-ignore-line
 		}
 		return $oxymel->to_string();
 	}
@@ -269,10 +282,10 @@ protected function comment_meta( $comment ) {
 		}
 		$oxymel = new WP_Export_Oxymel();
 		foreach ( $metas as $meta ) {
-			$oxymel->tag( 'wp:commentmeta' )->contains
+			$oxymel->tag( 'wp:commentmeta' )->contains // @phpstan-ignore-line
 				->tag( 'wp:meta_key', $meta->meta_key )
 				->tag( 'wp:meta_value' )->contains->cdata( $meta->meta_value )->end
-			->end;
+			->end; // @phpstan-ignore-line
 		}
 		return $oxymel;
 	}
diff --git a/src/WP_Export_XML_Over_HTTP.php b/src/WP_Export_XML_Over_HTTP.php
index f4dee588..a0332b2f 100644
--- a/src/WP_Export_XML_Over_HTTP.php
+++ b/src/WP_Export_XML_Over_HTTP.php
@@ -1,9 +1,20 @@
 <?php
 
-
 class WP_Export_XML_Over_HTTP extends WP_Export_Base_Writer {
+	/**
+	 * @var string
+	 */
+	private $result;
+
+	/**
+	 * @var string
+	 */
 	private $file_name;
 
+	/**
+	 * @param WP_Export_WXR_Formatter $formatter
+	 * @param string                  $file_name
+	 */
 	public function __construct( $formatter, $file_name ) {
 		parent::__construct( $formatter );
 		$this->file_name = $file_name;
diff --git a/src/WP_Map_Iterator.php b/src/WP_Map_Iterator.php
index 9d2d0b3e..b68a618e 100644
--- a/src/WP_Map_Iterator.php
+++ b/src/WP_Map_Iterator.php
@@ -1,6 +1,18 @@
 <?php
 
+/**
+ * @extends IteratorIterator<mixed, mixed, \Iterator<mixed>>
+ */
 class WP_Map_Iterator extends IteratorIterator {
+	/**
+	 * @var callable
+	 */
+	private $callback;
+
+	/**
+	 * @param \Iterator $iterator
+	 * @param callable $callback
+	 */
 	public function __construct( $iterator, $callback ) {
 		$this->callback = $callback;
 		parent::__construct( $iterator );
diff --git a/src/WP_Post_IDs_Iterator.php b/src/WP_Post_IDs_Iterator.php
index b05c6f12..4560ccc3 100644
--- a/src/WP_Post_IDs_Iterator.php
+++ b/src/WP_Post_IDs_Iterator.php
@@ -1,11 +1,44 @@
 <?php
 
+/**
+ * @implements \Iterator<int, object{}>
+ */
 class WP_Post_IDs_Iterator implements Iterator {
+	/**
+	 * @var \wpdb
+	 */
+	private $db;
+
+	/**
+	 * @var int
+	 */
 	private $limit = 100;
+
+	/**
+	 * @var int[]
+	 */
 	private $post_ids;
+
+	/**
+	 * @var int[]
+	 */
 	private $ids_left;
+
+	/**
+	 * @var object[]
+	 */
 	private $results = array();
 
+	/**
+	 * @var int
+	 */
+	private $index_in_results;
+
+	/**
+	 * @var int
+	 */
+	private $global_index;
+
 	public function __construct( $post_ids, $limit = null ) {
 		$this->db       = $GLOBALS['wpdb'];
 		$this->post_ids = $post_ids;
@@ -15,19 +48,23 @@ public function __construct( $post_ids, $limit = null ) {
 		}
 	}
 
+	#[\ReturnTypeWillChange]
 	public function current() {
 		return $this->results[ $this->index_in_results ];
 	}
 
+	#[\ReturnTypeWillChange]
 	public function key() {
 		return $this->global_index;
 	}
 
+	#[\ReturnTypeWillChange]
 	public function next() {
 		++$this->index_in_results;
 		++$this->global_index;
 	}
 
+	#[\ReturnTypeWillChange]
 	public function rewind() {
 		$this->results          = array();
 		$this->global_index     = 0;
@@ -35,6 +72,7 @@ public function rewind() {
 		$this->ids_left         = $this->post_ids;
 	}
 
+	#[\ReturnTypeWillChange]
 	public function valid() {
 		if ( isset( $this->results[ $this->index_in_results ] ) ) {
 			return true;
@@ -53,7 +91,8 @@ public function valid() {
 	private function load_next_posts_from_db() {
 		$next_batch_post_ids = array_splice( $this->ids_left, 0, $this->limit );
 		$in_post_ids_sql     = _wp_export_build_IN_condition( 'ID', $next_batch_post_ids );
-		$this->results       = $this->db->get_results( "SELECT * FROM {$this->db->posts} WHERE {$in_post_ids_sql}" );
+		$results             = $this->db->get_results( "SELECT * FROM {$this->db->posts} WHERE {$in_post_ids_sql}" );
+		$this->results       = is_array( $results ) ? $results : array();
 		if ( ! $this->results ) {
 			if ( $this->db->last_error ) {
 				throw new WP_Iterator_Exception( "Database error: {$this->db->last_error}" );