Python tkinter Entry widget validation error : TypeError: ‘str’ object is not callable

I am trying to use built-in input validation of Tkinter’s Entry widget but I can’t figure out why I receive a TypeError when entry validation is triggered. (regardless of what kind of event triggers the validation). In the simple code below, I am trying to validate only numerical input from the user. Alphabetical characters must disable the Apply button. But as soon as you enter ANY character, a “TypeError: ‘str’ object is not callable” is raided. Any ideas why?!