matplotlib: AttributeError: 'AxesSubplot' object has no attribute 'add_axes'

Not sure exactly sure how to fix the following attribute error:

AttributeError: 'AxesSubplot' object has no attribute 'add_axes'

The offending problem seems to be linked to the way I have set up my plot:

gridspec_layout = gridspec.GridSpec(3,3)
pyplot_2 = fig.add_subplot(gridspec_layout[2])

ax = WCSAxes(fig, [0.1, 0.1, 0.8, 0.8], wcs=wcs)
pyplot_2.add_axes(ax)

Does anybody know how to solve this? Many thanks.