diff --git a/base/src/main/java/org/bitcoinj/base/exceptions/package-info.java b/base/src/main/java/org/bitcoinj/base/exceptions/package-info.java
new file mode 100644
index 00000000000..9a8c688830a
--- /dev/null
+++ b/base/src/main/java/org/bitcoinj/base/exceptions/package-info.java
@@ -0,0 +1,23 @@
+/*
+ * Copyright by the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/**
+ * Exceptions thrown by classes in bitcoinj base.
+ */
+@NullMarked
+package org.bitcoinj.base.exceptions;
+
+import org.jspecify.annotations.NullMarked;
diff --git a/base/src/main/java/org/bitcoinj/base/internal/Stopwatch.java b/base/src/main/java/org/bitcoinj/base/internal/Stopwatch.java
index fdda3c11d0b..4d8ef87d4a4 100644
--- a/base/src/main/java/org/bitcoinj/base/internal/Stopwatch.java
+++ b/base/src/main/java/org/bitcoinj/base/internal/Stopwatch.java
@@ -16,6 +16,8 @@
  * limitations under the License.
  */
 
+import org.jspecify.annotations.Nullable;
+
 import java.time.Duration;
 import java.time.Instant;
 import java.time.temporal.ChronoUnit;
@@ -33,6 +35,7 @@
  */
 public class Stopwatch implements TemporalAmount {
     private final Instant startTime;
+    @Nullable
     private Instant stopTime = null;
 
     /**
diff --git a/base/src/main/java/org/bitcoinj/base/internal/package-info.java b/base/src/main/java/org/bitcoinj/base/internal/package-info.java
new file mode 100644
index 00000000000..e9be49e8bed
--- /dev/null
+++ b/base/src/main/java/org/bitcoinj/base/internal/package-info.java
@@ -0,0 +1,23 @@
+/*
+ * Copyright by the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/**
+ * Classes for internal use only (in both {@code bitcoinj-base} and {@code bitcoinj-core}.)
+ */
+@NullMarked
+package org.bitcoinj.base.internal;
+
+import org.jspecify.annotations.NullMarked;
diff --git a/base/src/main/java/org/bitcoinj/base/package-info.java b/base/src/main/java/org/bitcoinj/base/package-info.java
index 5bdce928c16..6053b5b1df3 100644
--- a/base/src/main/java/org/bitcoinj/base/package-info.java
+++ b/base/src/main/java/org/bitcoinj/base/package-info.java
@@ -18,4 +18,7 @@
  * The {@code base} package provides foundational types for <b>bitcoinj</b>. This package
  * has no dependencies other than {@code slf4j-api} and {@code jspecify}.
  */
+@NullMarked
 package org.bitcoinj.base;
+
+import org.jspecify.annotations.NullMarked;
diff --git a/base/src/main/java/org/bitcoinj/base/utils/MonetaryFormat.java b/base/src/main/java/org/bitcoinj/base/utils/MonetaryFormat.java
index ee7f5ffbc93..25c14fca92d 100644
--- a/base/src/main/java/org/bitcoinj/base/utils/MonetaryFormat.java
+++ b/base/src/main/java/org/bitcoinj/base/utils/MonetaryFormat.java
@@ -18,6 +18,7 @@
 
 import org.bitcoinj.base.Coin;
 import org.bitcoinj.base.Monetary;
+import org.jspecify.annotations.Nullable;
 
 import java.math.BigDecimal;
 import java.math.RoundingMode;
@@ -78,10 +79,10 @@ public final class MonetaryFormat {
     private final char zeroDigit;
     private final char decimalMark;
     private final int minDecimals;
-    private final List<Integer> decimalGroups;
+    private final @Nullable List<Integer> decimalGroups;
     private final int shift;
     private final RoundingMode roundingMode;
-    private final String[] codes;
+    private final String @Nullable[] codes;
     private final char codeSeparator;
     private final boolean codePrefixed;
 
@@ -342,7 +343,7 @@ public MonetaryFormat(boolean useSymbol) {
     }
 
     private MonetaryFormat(char negativeSign, char positiveSign, char zeroDigit, char decimalMark, int minDecimals,
-            List<Integer> decimalGroups, int shift, RoundingMode roundingMode, String[] codes,
+            @Nullable List<Integer> decimalGroups, int shift, RoundingMode roundingMode, String @Nullable[] codes,
             char codeSeparator, boolean codePrefixed) {
         this.negativeSign = negativeSign;
         this.positiveSign = positiveSign;
@@ -513,6 +514,7 @@ private long parseValue(String str, int smallestUnitExponent) {
     /**
      * Get currency code that will be used for current shift.
      */
+    @Nullable
     public String code() {
         if (codes == null)
             return null;
diff --git a/base/src/main/java/org/bitcoinj/base/utils/package-info.java b/base/src/main/java/org/bitcoinj/base/utils/package-info.java
new file mode 100644
index 00000000000..0e2a1192b7f
--- /dev/null
+++ b/base/src/main/java/org/bitcoinj/base/utils/package-info.java
@@ -0,0 +1,23 @@
+/*
+ * Copyright by the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/**
+ * bitcoinj utility classes.
+ */
+@NullMarked
+package org.bitcoinj.base.utils;
+
+import org.jspecify.annotations.NullMarked;