det coordinates not working

[Russell-Ryan]

I am trying to do coordinate conversions between det and sci reference frames for HST instruments. pysiaf gives this error on this MWE:

import pysiaf

s = pysiaf.Siaf('hst')
A = s['JWFC']

v2,v3 = A.det_to_tel(1000,1000)

It says:

raise TypeError('Angle has to be a float. It is of type {} and has the value {}'.format(
TypeError: Angle has to be a float. It is of type <class 'NoneType'> and has the value None

I've tried a few other apertures, and it gives the same error.