Object arrays are not currently supported

I am using code:
def front_back(a, b):
 # +++your code here+++
  if len(a) % 2 == 0 && len(b) % 2 == 0:
    return a[:(len(a)/2)] + b[:(len(b)/2)] + a[(len(a)/2):] + b[(len(b)/2):] 
  else:
    #todo! Not yet done. 
  return
I'm getting an error in the IF conditional. What am I doing wrong?