Bogus assertion in multixact.c?

There's an assertion in multixact.c, MultiXactIdExpand function, line 273:
       Assert(!TransactionIdEquals(multi, xid));

where multi is a MultiXactId and xid is a TransactionId.

Isn't this bogus? If I understand the code correctly, multixactids and 
regular xids live in completely separate id spaces. Therefore a comparison 
between them makes no sense.

- Heikki