diff --git a/src/proto/os_win32.pro b/src/proto/os_win32.pro
index a64c863..ca67146 100644
--- a/src/proto/os_win32.pro
+++ b/src/proto/os_win32.pro
@@ -65,4 +65,5 @@
 void used_file_arg(char *name, int literal, int full_path, int diff_mode);
 void set_alist_count(void);
 void fix_arg_enc(void);
+int mch_setenv(char *var, char *value, int x);
 /* vim: set ft=c : */