diff --git a/2.10/Source/Mutate.pas b/2.10/Source/Mutate.pas
index 66d07b4..fbea05a 100644
--- a/2.10/Source/Mutate.pas
+++ b/2.10/Source/Mutate.pas
@@ -190,6 +190,8 @@ procedure TMutateForm.ShowMutants;
 var
   i: integer;
 begin
+  if Visible = false then exit;
+
   Updating := true;
   for i := 1 to 8 do
   begin