Problem to save a particular record CRM2011

Hi,

I am facing problems while trying to save a new record onto CRM. Strangely, this only happens for a particular entity. I have tested many other entities and they all are able to store data but this one in particular does not. I usually receive a Null Reference Exception but when I debug it, I can see that the object is still instantiated.

What makes CRM trigger such exception?

Regards,
Demerson