Anthrotype helped me find the issue on on the FontTools repo: https://github.com/fonttools/fonttools/issues/1688 from fontTools.ufoLib.glifLib import GlifLibError from defcon import Font ufo = Font("YourFont.ufo") for layer in ufo.layers: for glyphName in layer.keys(): try: layer[glyphName] except GlifLibError: print(layer.name, glyphName)