Maya: “Unknown Nodes or Data”

I’ve just had the misfortune of trying to export a very basic model with Arnold Shaders from Maya as a .ma file. Upon trying this I get the fateful error.

A solution I’ve very quickly found is this python script from Fred Lewis (http://www.movingmedia.com/python-scripts-maya/)

# Unlock unknown types and delete them
import maya.cmds as mc
import sys
unknowns = mc.ls(type='unknown')
if unknowns:
    mc.lockNode(unknowns, lock=0)
    mc.delete(unknowns)
    unknowns = mc.ls(type='unknown')
    if not unknowns:
        sys.stdout.write('All unknown nodes have been deleted.')
    else:
        sys.stdout.write('Somehow, not all unknown nodes were deleted.')
else:
    sys.stdout.write('There are no unknown nodes in the scene.')