How to input a placeholding element if an error comes up?

Question

I have a list of dictionaries like this:

some_list = [{'a': 'Matt', 
              'b': 'Male',
              'c': '26'},

             {'a': 'Sherry',
              'b': 'Female,
              'c': '20',
              'd': 'NY'},

             {'a': 'Wally',
              'b': 'Male',
              'c': '20'},

             {'a': 'Ella',
              'b': 'Female',
              'c': '28'
             }]

I want to loop thru these dictionaries and append a list of values into another list like so:

big_list = []

for i in range(len(some_list)):
    entry = [some_list[i]['a'],
             some_list[i]['b'],
             some_list[i]['c'],
             some_list[i]['d']]
    big_list.append(entry)

but notice how not every entry in the dictionary has a d key.

Is there a way to input a NaN value or any element as a placeholder if that entry didn't have a d key?

Ideally the end result would look like this:

big_list = [['Matt', 'Male', '26', NaN],
            ['Sherry', 'Female', '20', 'NY']
            ['Wally', 'Male', '20', NaN]
            ['Ella', 'Female', '28', NaN]]