diff --git a/src/modules/pkpy.c b/src/modules/pkpy.c
index 87bc64d9..e7a7b015 100644
--- a/src/modules/pkpy.c
+++ b/src/modules/pkpy.c
@@ -65,6 +65,7 @@ static bool pkpy_setup_gc_debug_callback(int argc, py_Ref argv) {
     } else {
         heap->debug_callback = argv;
     }
+    py_newnone(py_retval());
     return true;
 }