Temperature conversion issue

I was very surprised by the result of the following I was expecting 0.001.

```
>>> import pint
>>> pint.__version__
'0.23'
>>> UR = pint.UnitRegistry
>>> q = UR.Quantity(1.0,'millidegree_Celsius')
>>> q.m_as(units="degree_Celsius")
-273.149
```

Am I missing something or is this a bug?