diff --git a/tests/server/sockfilt.c b/tests/server/sockfilt.c
index 42071a5bc4..6695d8486c 100644
--- a/tests/server/sockfilt.c
+++ b/tests/server/sockfilt.c
@@ -549,7 +549,7 @@ static DWORD WINAPI select_ws_wait_thread(LPVOID lpParameter)
     free(data);
   }
   else
-    return -1;
+    return (DWORD)-1;
 
   /* retrieve the type of file to wait on */
   type = GetFileType(handle);