diff --git a/src/if_mzsch.c b/src/if_mzsch.c
index 1861df4..37cebc5 100644
--- a/src/if_mzsch.c
+++ b/src/if_mzsch.c
@@ -849,7 +849,7 @@
 static int mz_threads_allow = 0;
 
 #if defined(FEAT_GUI_W32)
-static void CALLBACK timer_proc(HWND, UINT, UINT, DWORD);
+static void CALLBACK timer_proc(HWND, UINT, UINT_PTR, DWORD);
 static UINT timer_id = 0;
 #elif defined(FEAT_GUI_GTK)
 # if GTK_CHECK_VERSION(3,0,0)