str.extract fails when non-default index is present
Input:

```
import pandas as pd

values = ['11', '23', '45', '98']
noindex = pd.Series(data=values, index=None)
yesindex = pd.Series(data=values, index=list('abcd'))

extractno = noindex.str.extract('(\d)')
print extractno

extractyes = yesindex.str.extract('(\d)')
print extractyes
```

Output:

```
0    1
1    2
2    4
3    9
dtype: object
a    NaN
b    NaN
c    NaN
d    NaN
dtype: object
```
