Save DB Diagram in SQL 2008

I am getting a strange error I have never seen before.

This is a fairly new and normal DB. I can create a new diagram (there are no others yet - this is the first one), bring in tables, but when I go to save I get this error:

TITLE: Microsoft SQL Server Management Studio
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Cannot insert the value NULL into column 'diagram_id', table 'SQL2008_xxxxxx_maindb.dbo.sysdiagrams'; column does not allow nulls. INSERT fails.
The statement has been terminated.
The 'sp_creatediagram' procedure attempted to return a status of NULL, which is not allowed. A status of 0 will be returned instead. (.Net SqlClient Data Provider)
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BUTTONS:
OK
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any ideas?

Thanks!!