Limiting text input in a text entry field

[Jinjenator]

Hi!

This should be a straightforward thing but I can only partially get it working.
I'm trying to limit a text fields input to only numbers and integers -
I can get numbers only by using:

on keyDown theKey
  if theKey is not a number then beep
  else pass keyDown
end keyDown

but I can't get it to accept integers, even if I specify 'an integer' in above.

I think I'm missing something simple!

Cheers
Neil