diff --git a/historic/jam/src/builtins.c b/historic/jam/src/builtins.c
index d8014be95..b2d10674e 100644
--- a/historic/jam/src/builtins.c
+++ b/historic/jam/src/builtins.c
@@ -1669,6 +1669,7 @@ bjam_call(PyObject* self, PyObject* args)
 
     frame_free( inner );
 
+    Py_INCREF(Py_None);
     return Py_None;
 }