bug in my code

[sunnysude]

when i run the code on my laptop it runs fine. but when i try to submit online it detects a bug with online Type Error  . what can i do to correct the code. the code is suppose to check for isogram, raise a typeerror if non string input is supplied and also return a false if an empty string is entered .
my code 

def is_isogram (saw):
  cat = str(saw)
  while True:
    if len(cat) < 1 :
      return ("(argument, False)")
    elif not cat.isalpha():
      raise TypeError ("\'Argument should be a string'")
    else:
      for me in cat:
        cat = cat.lower()
        if cat.count(me) > 1:
          return (cat, False)
      return (cat, True)