[AstroPy] question about coordinate transformation

[Evangelia Samara]

Dear,

I am writing this email because I deal with a very peculiar problem (at
least, peculiar for me!).
I want to transform my coordinates from heliographic_stonyhurst to
heliographic_carrington in python 3, as follows:

import astropy.units as u
from astropy.coordinates import SkyCoord
from sunpy.coordinates import frames

c = SkyCoord(0*u.arcsec, 0*u.arcsec, obstime='2016-6-4T12:00:00',
frame=frames.HeliographicStonyhurst )
d = c.transform_to(frames.HeliographicCarrington)

But I get the error:

ConvertError: This transformation cannot be performed because the
HeliographicCarrington frame has observer=None.

The observer should not be "None" since I define the "obstime"
parameter above. Could you please explain me why this happens and how
it should be fixed?

BTW, it runs fine with Python 2 if i change "obstime" to "dateobs".

Thank you in advance,

Evangelia
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