This answer was deleted even though it answered the question:
Are you inserting the value null, or a zero length string ''?
A zero length string will be accepted with a NOT NULL constraint as it isn't null. If not can you please post your insert code too?
A very similiar answer was posted later and got accepted as the answer.
The post was from a 30 rep user who is presumably new, and was deleted by a moderator, that is, without any voting.