Calling ureg('-') causes an AssertionError

[catsclaw]

I'm reading in text files and trying to identify units in them. I just randomly discovered that calling ureg('-') throws an AssertionError rather than a pint.errors.UndefinedUnitError. It appears to do the same thing for other symbols, like ureg('*').

I could understand maybe throwing an error other than an UndefinedUnitError, but an AssertionError is clearly a bad choice here.