Cross-database limitations
Django doesn’t currently provide any support for foreign key or many-to-many relationships spanning multiple databases. If you have used a router to partition models to different databases, any foreign key and many-to-many relationships defined by those models must be internal to a single database.
Django – limitations-of-multiple-databases
Trouble
Same trouble. Bug in ForeignKey() class.
In validate() method.
Bug exists in v1.2, v1.3, v1.4rc1
Solution
Try this patch to solve it.