diff --git a/lcl/forms.pp b/lcl/forms.pp
index 115334503f..1320d37a1c 100644
--- a/lcl/forms.pp
+++ b/lcl/forms.pp
@@ -160,7 +160,7 @@ type
     Procedure SetFocus; override;
     function SetFocusedControl(Control : TWinControl): Boolean ; Virtual;
     Procedure FocusControl(Control : TWinControl);
-    Function  ShowModal : Integer;
+    Function ShowModal : Integer;
     property Active : Boolean read FActive;
     property BorderStyle : TFormBorderStyle read FBorderStyle write SetBorderStyle default bsSizeable;
     property Canvas: TControlCanvas read GetCanvas;
diff --git a/lcl/include/customform.inc b/lcl/include/customform.inc
index d1170ad224..d0b9388e40 100644
--- a/lcl/include/customform.inc
+++ b/lcl/include/customform.inc
@@ -932,8 +932,17 @@ end;
 {  TCustomForm ShowModal                                                       }
 {------------------------------------------------------------------------------}
 Function TCustomForm.ShowModal : Integer;
+{var
+  WindowList: Pointer;
+  SaveFocusCount: Integer;
+  SaveCursor: TCursor;
+  SaveCount: Integer;
+  ActiveWindow: HWnd;}
 begin
 //writeln('[TCustomForm.ShowModal] START ',Classname);
+  if Visible or not Enabled or (fsModal in FFormState) or
+    (FormStyle = fsMDIChild) then
+    raise EInvalidOperation.Create('TCustomForm.ShowModal impossible');
   { TODO : This has to be changed by WM_VISIBLECHANGED. Implement appropriate callback !!! }
   //Kill capture when opening another dialog
   if GetCapture <> 0 then SendMessage(GetCapture,LM_CANCELMODE,0,0);
@@ -958,6 +967,9 @@ end;
 { =============================================================================
 
   $Log$
+  Revision 1.39  2002/04/26 12:26:50  lazarus
+  MG: improved clean up
+
   Revision 1.38  2002/03/25 17:59:20  lazarus
   GTK Cleanup
   Shane