You might be reading TS 29.272 Section 18.104.22.168.3 Detailed behaviour of the HSS
If the Update Location Request is received over the S6a interface, and the subscriber has not any APN configuration,the HSS shall return a Result Code of DIAMETER ERROR UNKNOWN EPS SUBSCRIPTION.
If the Update Location Request is received over the S6d interface, and the subscriber has neither an APN configuration profile nor GPRS subscription data, the HSS shall return a Result Code of DIAMETER ERROR UNKNOWN EPS SUBSCRIPTION.
Error Code *DIAMETER ERROR UNKNOWN EPS SUBSCRIPTION (5420):*
This result code shall be sent by the HSS to indicate that no EPS subscription is associated with the IMSI.
This error is a permanent failure. Errors that fall within the Permanent Failures category shall be used to inform the peer that the request has failed and should not be attempted again. The Result-Code AVP values defined in Diameter Base Protocol RFC-3588/6733 shall be applied. When one of the result codes defined here is included in a response, it shall be inside an Experimental-Result AVP and the Result-Code AVP shall be absent.