Calculation of a DH secret key

From: Nick (Nick_at_discussions.microsoft.com)
Date: 06/29/04 

Date: Tue, 29 Jun 2004 07:08:26 -0700

While using MS DSS and DH CSP for calculation of a DH secret key I've obtained the following results.

AlgId of imported BLOB AlgId of hPubKey Result

AT_KEYEXCHANGE AT_SIGNATURE false (error 0x80090003 "Bad Key")
AT_KEYEXCHANGE AT_KEYEXCHANGE true
AT_SIGNATURE AT_SIGNATURE true
AT_SIGNATURE AT_KEYEXCHANGE true

Is there anyone who can answer me the following question:
Why does the calculation fail for the first case?

Please note that obtained results do not depend on a version of the imported BLOB.

Thanks,

Nick