Oracle Message ORA-28545 - error diagnosed by Net8 when connecting to an agent

ERRORerror diagnosed by Net8 when connecting to an agent
CAUSEAn attempt to call an external procedure or to issue SQL to a non-Oracle system on a Heterogeneous Services database link failed at connection initialization. The error diagnosed by Net8 NCR software is reported separately.
ACTIONRefer to the Net8 NCRO error message. If this isnt clear, check connection administrative setup in tnsnames.ora and listener.ora for the service associated with the Heterogeneous Services database link being used, or with extproc_connection_data for an external procedure call.