Jython Type Error - Maximo automation script

I'm using the below script to see if 'ttype' is changed so I can reset the value of 'tclass'

if ttype != None & ttype != '':
  tclass=None

However I'm receiving the following error when triggering the script in the application:

TypeError: unsupported operand type(s) for &: 'NoneType' and 'unicode' in <script> at line number 1

I'm new to python/jython so any input would be helpful