diff --git a/Modules/main.c b/Modules/main.c
index 008b6a4..590104d 100644
--- a/Modules/main.c
+++ b/Modules/main.c
@@ -646,7 +646,7 @@
             if (fp == NULL) {
                 char *cfilename_buffer;
                 const char *cfilename;
-                cfilename_buffer = _Py_wchar2char(filename);
+                cfilename_buffer = _Py_wchar2char(filename, NULL);
                 if (cfilename_buffer != NULL)
                     cfilename = cfilename_buffer;
                 else