9.2 letter counts

error

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-160-81dcb3a1b66b> in <module>()
----> 1 _ = words.apply(update_counts)

/Users/gonzo/.pyenv/versions/miniconda3-latest/lib/python3.5/site-packages/pandas/core/series.py in apply(self, func, convert_dtype, args, **kwds)
   2235             values = lib.map_infer(values, boxer)
   2236 
-> 2237         mapped = lib.map_infer(values, f, convert=convert_dtype)
   2238         if len(mapped) and isinstance(mapped[0], Series):
   2239             from pandas.core.frame import DataFrame

pandas/src/inference.pyx in pandas.lib.map_infer (pandas/lib.c:63043)()

<ipython-input-159-487c851bece5> in update_counts(val)
      2 def update_counts(val):
      3     global counts
----> 4     for let in val:
      5         let = let.lower()
      6         count = counts.get(let, 0) + val.count(let)

TypeError: 'float' object is not iterable

fix

words = pd.Series.from_csv(filename, index_col=None, encoding='utf-8')
# add this line
words.dropna(inplace=True);
...