ForeignKey to a model with a CharField primary key produces unexpected results for an empty-string PK value

[Ubercore]

class ModelA(models.Model):
    code = models.CharField(max_length=50, primary_key=True)

class ModelB(models.Model):
    model_a = models.ForeignKey(ModelA, blank=True, null=True, max_length=50)

In this case, if there's an instance of ModelA such that instance_a.code == '', ModelB will not be able to reference it properly. Upon save, db.models.fields.related.ForeignKey will change model_a_id from '' to NULL. While this is an odd case, and I don't particularly like the situation of having an empty string as a primary key, I think Django should at least be able to handle it. The offending code is at ​https://github.com/django/django/blob/master/django/db/models/fields/related.py#L987.

[anonymous]

You cannot have a empty primary_key.
Can you provide a failing test for this ?

[Ubercore]

Interesting. This project must have gotten into an odd state with migrations. I'll investigate a bit more, but it does look like an invalid ticket. Thanks.