diff --git a/src/os_mswin.c b/src/os_mswin.c
index 60c5a81..94cf3a9 100644
--- a/src/os_mswin.c
+++ b/src/os_mswin.c
@@ -1227,8 +1227,8 @@
  * Wait for another process to Close the Clipboard.
  * Returns TRUE for success.
  */
-    int
-vim_open_clipboard()
+    static int
+vim_open_clipboard(void)
 {
     int delay = 10;