Setting float_series[bool_index] = None errors
``` python
s = pd.Series(range(10)).astype(float)
s[s > 8] = None
s
```

```
/Users/datacaliber/Dropbox/Projects/trading/python/externals/pandas/pandas/core/internals.py in create_block(v, m, n, item, reshape)
    596                 # change the dtype
    597                 if nv is None:
--> 598                     dtype, _ = com._maybe_promote(n.dtype)
    599                     nv = v.astype(dtype)
    600                     try:

AttributeError: 'NoneType' object has no attribute 'dtype'
```

This is on latest master.
