Mark _Py_char2wchar() input argument as constant
diff --git a/Include/fileutils.h b/Include/fileutils.h
index a232460..cb15936 100644
--- a/Include/fileutils.h
+++ b/Include/fileutils.h
@@ -6,7 +6,7 @@
 #endif
 
 PyAPI_FUNC(wchar_t *) _Py_char2wchar(
-    char *arg);
+    const char *arg);
 
 PyAPI_FUNC(char*) _Py_wchar2char(
     const wchar_t *text);