Non-Blob. Identity problems fields. Delphi 4 & MSSQL 6.5

I'm programming a C/S aplicattion with Delphi 4 (sp. 2) and MSSQL Server

I have a problem when I want insert a record in a table with an Identity

field, I get
"Non-Blob Column required to perform this operation".
If I change the field kind (not identity) I dont get the problem but I
need this kind of  field.
If you know something about this matter, be sure I will thank you
for ever.
Please write me.

Thank You.