The new namespace map printing is causing a failure in printing bean maps (which are proxies that don't support empty):
Cause: The internal lift-ns function calls empty on the map too early (here it doesn't need to call it at all).
Proposed: Defer calling (empty m) until we know map has namespace keywords and namespace maps will be used for printing.
Patch: clj-2027.patch (note that into is not used in the change b/c it has not yet been defined at this point)
Just as an added datapoint: I ran in to this same problem with a different custom map type.