Error in find-database

The tutorial shows calling 'disconnect' in succession when there are 
multiple databases open. Well, my failures in the ODBC tests left
connections open (which is in itself a bit of an error). I kept calling 
'disconnect', but when there were no more connections I got this error:

        Error: NIL fell through ETYPECASE expression.
        Wanted one of (CLSQL-SYS:DATABASE STRING).
          1 (abort) Return to level 0.
          2 Return to top loop level 0.

The problem is that the first thing 'disconnect' does is to call 
'find-database', passing it (in effect) *default-database*. 
'find-database' passes that value into an etypecase, not expecting a nil 
value--which, of course, *default-database* might be.