diff --git a/lib/CrEOF/Spatial/ORM/Query/AST/Functions/AbstractSpatialDQLFunction.php b/lib/CrEOF/Spatial/ORM/Query/AST/Functions/AbstractSpatialDQLFunction.php
index cd8b0d3f..83557aa9 100644
--- a/lib/CrEOF/Spatial/ORM/Query/AST/Functions/AbstractSpatialDQLFunction.php
+++ b/lib/CrEOF/Spatial/ORM/Query/AST/Functions/AbstractSpatialDQLFunction.php
@@ -90,7 +90,7 @@ public function parse(Parser $parser)
      *
      * @return string
      */
-    public function getSql(SqlWalker $sqlWalker)
+    public function getSql(SqlWalker $sqlWalker): string
     {
         $this->validatePlatform($sqlWalker->getConnection()->getDatabasePlatform());