diff --git a/lcl/comctrls.pp b/lcl/comctrls.pp
index 4762700851..4b0cc6afed 100644
--- a/lcl/comctrls.pp
+++ b/lcl/comctrls.pp
@@ -219,7 +219,7 @@ type
     property Width stored False;
   end;
 
-  TGetDockCaptionEvent = procedure(Sender: TObject; AControl: TControl; ACaption: String) of Object;
+  TGetDockCaptionEvent = procedure(Sender: TObject; AControl: TControl; var ACaption: String) of Object;
 
   { TPageControl }