Drop a user leaves permissions!

I'm using PostgreSQL and I've been working with permissions.
I've tried the following:

CREATE USER ben;
GRANT ALL ON table1 TO ben;
DROP USER ben;

The permissions get left to the table as in :

     Relation    |      Access permissions
------------------------------------------------------
table1            | {"=","51=arwR"}

I assume that 51 is the ID of the user who was dropped.
Is this a bug?  How should I clean up the permissions?

thanks

Ben