Do you have any idea why i'm getting just half of the test cases? def morganAndString(a, b):

# Write your code here
morgan_str=''
while len(a) > 0 and len(b) > 0:
    if a <= b:
        morgan_str+=a[0]
        a= a[1:]
    else:
        morgan_str+=b[0]
        b= b[1:]

if len(a) == 0:
    morgan_str+=b
else:
    morgan_str+=a
return morgan_str