Resolved: Error "out of index" even with modulus

My code is below:

def encrypt(text,shift):
    alphabet='abcdefghijklmnopqrstuwxyz'
    output=''
    text=text.lower()
    for char in text:
        if char not in alphabet:
            output=output+char
        else:
            pos=alphabet.find(char)
            output=output+alphabet[(pos+shift)%26]
    return output

If shift is 5 and there is a "u" inside the text, it gives me the error "string index out of range" in line "output=output+alphabet[(pos+shift)%26]". Why can't I write "(pos+shift)%26" instead of calling a function that does the same? Thank you