diff --git a/lcl/interfaces/win32/win32wscalendar.pp b/lcl/interfaces/win32/win32wscalendar.pp
index da47fd8dd9..7a38141bd8 100644
--- a/lcl/interfaces/win32/win32wscalendar.pp
+++ b/lcl/interfaces/win32/win32wscalendar.pp
@@ -76,7 +76,7 @@ begin
     pClassName := 'SysMonthCal32';
     WindowTitle := StrCaption;
     Flags := WS_CHILD or WS_VISIBLE;
-    SubClassWndProc := nil;
+    SubClassWndProc := @WindowProc;
   end;
   // create window
   FinishCreateWindow(AWinControl, Params, false);