dbms_alert.register fails 12.1.0.2

I have also posted this in the database utilities space - apologies if this breaches etiquette

We have an issue where execution of one of our packages fails with an ora-00022.

As part of its processing the package calls DBMS_ALERT.REGISTER

Stripped down to the absolute minimum that failing test is

SQL> exec dbms_alert.register('t');

BEGIN dbms_alert.register('t'); END;

*

ERROR at line 1:

ORA-00022: invalid session ID; access denied

ORA-06512: at "SYS.DBMS_SESSION", line 240

ORA-06512: at "SYS.DBMS_ALERT", line 79

ORA-06512: at "SYS.DBMS_ALERT", line 103

ORA-06512: at line 1

Process ID: 2008

Session ID: 75 Serial number: 4122