Triple nested dictionary comprehension?

Suppose I have a

, like this:

import pandas as pd

s = pd.Series(["hello go home bye bye", "you can't always get", "what you waaaaaaant", "apple banana carrot munch 123"])

I want to create a dictionary with individual characters as keys, and their frequencies as values. Creating these dictionaries for words in the past has been easy with the help of :

from collections import Counter

c = Counter(word for row in s for word in row.lower().split())

However, I'm trying now to store individual characters and am having some issues with triple-nested dict comprehensions. Here's what I have:

c = Counter((letter for letter in word) for word for row in s for word in row.lower().split())

Which gives me a syntax error. How can I make the equivalent of the following loop in one line?

d = {}

for row in s:

    for word in row.lower().split():

        for letter in word:

            d[letter] += 1