FC6: Problem with signatures while upgrading

Upgrading FC6 using yumex produces the following error box:
        /nError checking package signatures:
        exceptions.AttributeError
        
The following extract from the output log indicates that something may
be wrong with the repo, especially since I don't have this problem when
upgrading only packages in the extras repo.
        ...
        ---> Package tar.x86_64 2:1.15.1-21.fc6 set to be updated
        --> Running transaction check
        Downloading Packages:
        Checking GPG Signatures:
        Public key for qt-3.3.7-0.1.fc6.x86_64.rpm is not installed
        Retrieving GPG key from
        file:///etc/pki/rpm-gpg/RPM-GPG-KEY-fedora
        ...
        
Any ideas what's going on?

Thanks - jon