This is a bug in Tk::DialogBox, you should report it here. To test run widget, under "Common Dialogs" click on "1. Message boxes.", when "messageBox Demo" pops up, choose Type OkCancel, then click "Message Box" button, and perform the test (tab over to cancel button, hit enter). Default is "Tk::DialogBox", and selecting Cancel and hitting enter doesn't work (falsely returns that you selected Ok). If you click radio button to switch to Tk::MsgBox , and perform test again, you will get Cancel as expected.