How to handle blank input by a user?

I have a UserForm TextBox and I want the user to enter an integer (in my program I assign the text box to an integer variable).  I use a case to evaluate different values for the text box but I cannot get my case to handle a blank input, and what about the use of a alpha character or string?  How can I handle these events?  I am very new to this stuff so please go easy on me...

Thanks!

SolidworksApi macros