The Most Wanted Letter Misson Error

Sorry for my English. My code:

def checkio(a):
    b = a.lower()
    d = {i : b.count(i) for i in b.lower() if i.isalpha()}
    m = max(d, key=d.get)
    return min(d) if m == 1 else m

I tried on Output (Python console), and the result is exactly. But when I run the test, It's wrong.