diff --git a/src/tools/qtprotobufgen/Qt6ProtobufToolsMacros.cmake b/src/tools/qtprotobufgen/Qt6ProtobufToolsMacros.cmake
index 9973f1eb..560fd3c8 100644
--- a/src/tools/qtprotobufgen/Qt6ProtobufToolsMacros.cmake
+++ b/src/tools/qtprotobufgen/Qt6ProtobufToolsMacros.cmake
@@ -197,6 +197,9 @@ function(_qt_internal_protoc_generate target generator output_directory)
     )
 
     get_target_property(proto_files ${target} _qt_internal_proto_files)
+    if(NOT proto_files)
+        set(proto_files "")
+    endif()
     list(APPEND proto_files "${arg_PROTO_FILES}")
     list(REMOVE_DUPLICATES proto_files)
     set_target_properties(${target} PROPERTIES _qt_internal_proto_files "${proto_files}")