Foreign key reference could not be resolved immediately after defining

Question

The following code

@schema
class A(dj.Manual):
    definition = """
    a: varchar(1)
    """
    
a = A()
a.insert1(["a"], skip_duplicates=True)
    
@schema
class B(dj.Manual):
    definition = """
    b: varchar(1)
    """    
    
@schema
class C(dj.Computed):
    definition = """
    -> A
    -> B
    """
    
    def _make_tuples(self, key):
        self.insert1(key)

raises DataJointError: Foreign key reference A could not be resolved. On my machine just this toy example reproduces this error -- what could possibly be going on, it is defined right there!