If you're going to do this there should be some mention in the docstring as to what this means e.g. it's used for coordinates containing strings. It might be better to use ```'no_unit'``` as 'no unit', 'no-unit' and 'no_unit' all result in ```iris.unit.Unit('no_unit')```. Feel free to tackle ```iris.unit.Unit(None)```/```iris.unit.Unit('unknown')``` while you are at it :wink: