Update with multple DB Links

We are trying to do an update statement from one database that uses links to two other databases. i.e. from DB3 we run the following statement:

update table_a@DB1  set sent = '05-JAN-41'
    where sent is null and valuex in
    (select value from table_b@DB2 where <whatever>);

The error returned is ORA-02019: connection description for remote database not found.

All permissions are correct, all links work if we run individual statements, it is only when we run them together that we get the error.  DB3 is 11.1.0.7, DB1 and DB2 are 10.2.0.5.  Is it possible to use more than one link in a single statement like that?  If so, what do we need to do to get it to work?  We have tried public links, private links and synonyms.