invFunc is optional and can be None. Instead of invFunc (the parameter) invReduceFunc (a local function) was checked for trueness (that is, not None, in this context). A local function is never None,
thus the case of invFunc=None (a common one when inverse reduction is not defined) was treated incorrectly, resulting in loss of data.
In addition, the docstring used wrong parameter names, also fixed.
Author: David Tolpin <david.tolpin@gmail.com>
Closes#9775 from dtolpin/master.
599a8c6e2b