E-mail 2.0 Template - mktoNumber Variable Maximum Value Error

I get some strange behavior for a mktoNumber field I'm using in a template I've built. When I try to update the default number, it states that "the maximum value for this field is 0". (I haven't set a maximum or a minimum for that variable.)

I can set the number to whatever I like and save the changes, so it actually works fine, but I'm concerned I'm going to hear from users. Is this a known bug or am I doing something wrong with my code?