Unknown error during update of property leaving out datatype

The following causes an "Unknown server error":

rt = db.RecordType("A")
r = db.Record()
r.add_parent(rt)
p = db.Property(name="B", datatype=db.INTEGER)
r.add_property(name="B", value=5)
db.Container().extend([rt, r, p]).insert()

r2 = db.Record(id=r.id).retrieve()
r2.remove_property("B")
r2.add_property(name="B", id=209, value=7)
r2.update()

Interestingly, I can leave out the datatype when adding the property to the record on insertion. On update it causes an unknown server error.