Pandas 0.21.0rc1 issue
Noticed a stack trace with 0.21.0rc1 caused by the as_matrix() function.
The only difference between each run was the Pandas version.

```
0.21.0rc1
data.as_matrix([['y', 'x']])
[[ nan  nan]
 [ nan  nan]]
```

```
0.20.3
data.as_matrix([['y', 'x']])
[[  11.56838028699999654236 -10.41327047200000777138]
 [  11.56846775199999655115 -10.41368445399999131951]]
```
