diff --git a/Python/fileutils.c b/Python/fileutils.c
index 3cf8b7a..32aeea4 100644
--- a/Python/fileutils.c
+++ b/Python/fileutils.c
@@ -1289,7 +1289,8 @@
     if (wpath == NULL)
         return NULL;
 
-    usize = MultiByteToWideChar(CP_ACP, 0, mode, -1, wmode, sizeof(wmode));
+    usize = MultiByteToWideChar(CP_ACP, 0, mode, -1,
+                                wmode, Py_ARRAY_LENGTH(wmode));
     if (usize == 0) {
         PyErr_SetFromWindowsErr(0);
         return NULL;