Packaging error: Graph is linked to private object(s) in an external package

Does anyone know what this error (during packaging) means ?
The bp is empty and derived from a c++ class.


Can't save /.../Weapon/BP_Weapon.uasset: Graph is linked to private object(s) in an external package.
External Object(s):
WeaponStateFiring_0
WeaponStateFiring_1

I would expect to get a compiler error when there are access errors.