Don't understand why this code isn't working

The objective is this: You're doing great! Just one more task but it's a bigger one. Right now, we turn everything into a float. That's great so long as we're getting numbers or numbers as a string. We should handle cases where we get a non-number, though. Add a try block before where you turn your arguments into floats. Then add an except to catch the possible ValueError. Inside the except block, return None. If you're following the structure from the videos, add an else: for your final return of the added floats.

def add(one, two):
    try:
        add = (one + two)
    except:
        return None
    else:
        return(float(one) + float(two))

I've tried this exact code in a separate text editor and it works perfectly fine when I call the function with any given arguments. I don't get why I'm getting an error in the browser. Am I missing something?

def add(number, letter):
    try:
        add = (number + letter)
    except:
        return None
    else:
        return(float(number) + float(letter))