diff --git a/src/analysis/processing/qgsalgorithmunion.cpp b/src/analysis/processing/qgsalgorithmunion.cpp
index 8e4e7d12fc4..f16d56834d0 100644
--- a/src/analysis/processing/qgsalgorithmunion.cpp
+++ b/src/analysis/processing/qgsalgorithmunion.cpp
@@ -65,7 +65,8 @@ QVariantMap QgsUnionAlgorithm::processAlgorithm( const QVariantMap &parameters,
     throw QgsProcessingException( invalidSourceError( parameters, QStringLiteral( "INPUT" ) ) );
 
   std::unique_ptr< QgsFeatureSource > sourceB( parameterAsSource( parameters, QStringLiteral( "OVERLAY" ), context ) );
-  // sourceB is optional so we do not throw an error if it is not a valid source
+  if ( parameters.value( QStringLiteral( "OVERLAY" ) ).isValid() && !sourceB )
+    throw QgsProcessingException( invalidSourceError( parameters, QStringLiteral( "OVERLAY" ) ) );
 
   QgsWkbTypes::Type geomType = QgsWkbTypes::multiType( sourceA->wkbType() );