In current versions of Juju old-style, non multi-series charms (those charms that don’t declare which series they support in their meta-data files but instead in their URLs) acted as wild-cards with respect to relations. That is, old-style charms could effectively relate to any other charm regardless of OS series restrictions. This has been updated in that old-style charms may now relate only to charms that support their series. This is the way multi-series charms work with the difference being old-style charms support only one series. If this rule breaks a bundle or otherwise the way to move forward is to update the offending charm to a multi-series charm.