InfoPath Dev

I have a problem, I have a field which is by default null

I can type a number in this field and infopath accepts it

I can set it back to null (or change it to another number) either by pressing delete or using code, infopath also accepts this

What I cant do is change it from null to a number using code

I have read about xsi:nil but i dont think that the problem because I can set the field to null manually and using code

I have just noticed if I set the field to null using code I get a red star, this is the only time I get one