Setting Quantity item inconsistent

As expected

``` python
In [42]: test = np.arange(1000) * u.km

In [43]: test[300] = 1000
ERROR: UnitsError: '' (dimensionless) and 'km' (length) are not convertible [astropy.units.quantity]

UnitsError: '' (dimensionless) and 'km' (length) are not convertible
```

but 

``` python
In [44]: test[300] = 0.0

In [45]: test[300]
Out[45]: <Quantity 0.0 km>
```

@mhvk any ideas?
