Affirming the Consequent

when i go home sometime I am alone by that I mean none of my home mates are in the house and sometimes they both are inside so I am not alone.

• 
  If they are not inside they are outside.
  


• They are outside
  


• Therefore, they are not inside.
  

The above argument may be valid, but only if the claim "if he's outside, then he's not inside" follows from the first premise. Even in such a case, however, the validity stems not from affirming the consequent, but from the form modus ponens.