Convert strings with periods to uppercase - Python

I am trying to convert a series of markets to uppercase so I can match with a zip dictionary. When I loop through the string and convert to uppercase, it works fine, as so:

def uppercase(series):
    for string in series:
        print(string.upper())
    
uppercase(markets.County)

But when I try to append this to a new list or with list comprehension, it fails:

def uppercase(series):

    string_upper = []
    
    for string in series:
        string_upper.append(string.upper())
    
uppercase(markets.County)

With an attribute error that it can't convert the periods to uppercase. But it just did that in the code above.

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-25-2c8a689b8491> in <module>
      6         string_upper.append(string.upper())
      7 
----> 8 uppercase(markets.County)

<ipython-input-25-2c8a689b8491> in uppercase(series)
      4 
      5     for string in series:
----> 6         string_upper.append(string.upper())
      7 
      8 uppercase(markets.County)

AttributeError: 'float' object has no attribute 'upper'

This seems like a 2 second simple solution. Why is it failing when it has to append the new data?