pluck() on property derived from leftJoin

does anybody know why

        var Lat = table.rows( indexes ).data().pluck( 'Lat' );

works, but

        var Lat = table.rows( indexes ).data().pluck( 'fi_branch.lat' );

does not?
The result is undefinded.

Could it be that pluck() has a problem with joined tables?

The corresponding line from the php files would be:

        Field::inst( 'Lat' ),

and

        Field::inst( 'fi_branch.lat' ),