KeyError: '__dummy__'  for pd.crosstab in pandas
get ~~ KeyError: '**dummy**' ~~ when I run the following:

```
np.random.seed(seed = 99)
s = np.random.randint(1,10,200)
s = pd.Series(np.where(s > 9, np.nan, s))
s1 = s[:100]
s2 = s[100:]
pd.crosstab(s1, s2)

KeyError: '__dummy__
```
