Error message recommends unsupported option

[TomNicholas]

When I try to use the num2date function to compute "years since ..." it recommends that I can use common_years as the unit as long as I set calendar='noleap'. However this doesn't seem to work (with an opaque error), and is also inconsistent with the docstring for num2date which says

common_years since is allowed only for the 365_day calendar

Either the error message or the docstring must be incorrect.

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cftime 1.6.3

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python 3.10

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