Error: Bag named: "..." already exists in same angular scope... can't use dragulaService

[mg1075]

Can someone please explain possible reasons for why one would get the following error when trying to use dragulaService.options inside the link function of a custom directive that injects dragulaService and in the html template of the directive includes a bag named whatever-bag?
Error: Bag named: "whatever-bag" already exists in same angular scope.
Yet if I do not use the dragulaService, and instead listen with scope.$on, there is no error?

[mg1075]

Added a controller to the custom directive, and put the dragulaService call inside the controller, removing it from the directive's link function. The error went away.