Record already exists error when key's datatype is integer

Hi all,
When I try to insert a record with an Integer type primary key, the record already exists error occurs. If I change the primary key's datatype to Code it works properly. It seems as a strange situation and I want to consult you if this is a known issue or if there is something I'm not aware of.

I'm trying to insert record from .net using a list type page based on a custom table. (NAV version is 2009 R2 TR)

Thanks.

Ufuk