Keys don't print in python

[dellaert]

gtdynamics.internal.PoseKey(5,777)

TypeError                                 Traceback (most recent call last)
~/opt/miniconda3/lib/python3.7/site-packages/IPython/core/formatters.py in __call__(self, obj)
    700                 type_pprinters=self.type_printers,
    701                 deferred_pprinters=self.deferred_printers)
--> 702             printer.pretty(obj)
    703             printer.flush()
    704             return stream.getvalue()

~/opt/miniconda3/lib/python3.7/site-packages/IPython/lib/pretty.py in pretty(self, obj)
    392                         if cls is not object \
    393                                 and callable(cls.__dict__.get('__repr__')):
--> 394                             return _repr_pprint(obj, self, cycle)
    395 
    396             return _default_pprint(obj, self, cycle)

~/opt/miniconda3/lib/python3.7/site-packages/IPython/lib/pretty.py in _repr_pprint(obj, p, cycle)
    698     """A pprint that just redirects to the normal repr function."""
    699     # Find newlines and replace them with p.break_()
--> 700     output = repr(obj)
    701     lines = output.splitlines()
    702     with p.group():

TypeError: __repr__(): incompatible function arguments. The following argument types are supported:
    1. (self: gtdynamics.gtdynamics.DynamicsSymbol, s: str) -> str

Invoked with: <repr raised Error>

The issue seems to be that there is no default value for the string in the wrapper.