diff --git a/src/core/symbology/qgscategorizedsymbolrenderer.cpp b/src/core/symbology/qgscategorizedsymbolrenderer.cpp
index 37ba956dd6ab..530b9b4d3154 100644
--- a/src/core/symbology/qgscategorizedsymbolrenderer.cpp
+++ b/src/core/symbology/qgscategorizedsymbolrenderer.cpp
@@ -533,7 +533,7 @@ QString QgsCategorizedSymbolRenderer::filter( const QgsFields &fields )
 
   for ( const QgsRendererCategory &cat : qgis::as_const( mCategories ) )
   {
-    if ( cat.value() == "" )
+    if ( cat.value() == "" || cat.value().isNull() )
     {
       hasDefault = true;
       defaultActive = cat.renderState();