Unexpected Pandas Division Behavior
Test Code

```
import pandas as pd
import numpy as np
a = pd.DataFrame(np.random.randint(0,100,size=(100, 4)))
a.tail(1)/a.head(1)
```

Expected Behavior:
some float point value

Actual Behavior:

```
   0   1   2   3
0  NaN NaN NaN NaN
99 NaN NaN NaN NaN
```

Confirmed on OSX python3, Pandas v0.19.0 v0.18.1
