A little help

I tried this code but it is returnig the whole sentence instead of just the uppercases letters

def find_message(text: str) -> str:

a = 0
b = []
while a < len(text):
    if text[a].isupper:
        b.append(text[a])
        a = a + 1
    else:
        a = a + 1
else:
    return ''.join(b)