Deleting textbox bound to integer to leave a null value

Hi - this seems like an obvious problem but Ive trawled the internet but only found one solution with a link that didn't work.

I have a textbox on my form. This is bound to an integer in my table set to allow null values.

When I enter a number into the textbox and save, the table updates - fine.

However, when I delete the number to leave the textbox blank, the form wont let me move the focus. It stays stuck on the textbox until I enter a value such as 0. However, this will not be intuitive for the user who will just want to delete the value and leave it at that.

Clearing text in a textbox bound to a varchar works fine. Am I missing something obvious?

(also stack overflow threads are not opening in my web browser - is anyone else having difficulties?)

Thanks
Frank