I've had the same problem. When you don't set the default value and the user doesn't enter a value in the control, the dataset contains a null for the value (as expected since nothing was entered). Unfortunately, the MM data checking doesn't seem to handle this possiblity (since the data field is not nullable) and it throws an error.

I've taken the lazy way out and put in default values all over the place. I haven't taken the time to look for a better solution.

I hope this contributes to understanding the problem.

Jeff