diff --git a/windows/dialog.c b/windows/dialog.c
index 51e0363..b9edb20 100644
--- a/windows/dialog.c
+++ b/windows/dialog.c
@@ -1491,7 +1491,7 @@
  */
 BOOL16 WINAPI IsDialogMessage16( HWND16 hwndDlg, SEGPTR msg16 )
 {
-    LPMSG16 msg = PTR_SEG_TO_LIN(msg16);
+    LPMSG16 msg = MapSL(msg16);
     BOOL ret, translate, dispatch;
     INT dlgCode = 0;