While loop and If Statement

[farzankh]

Hello friends,
This is our assignment:
Write a function called stop_at_z that iterates through a list of strings. Using a while loop, append each string to a new list until the string that appears is “z”. The function should return the new list.

I have came up with this:

def stop_at_z(testlist):
    index = 0
    result = []
    while index < len(testlist):
        if "z" in testlist[index]:
            index += 1
            continue
        result.append(testlist[index])
        index += 1
    return result

But I receive error!
https://pasteboard.co/HYpOEuJ.png

Can somebody help?

Thank you