Forces on piston in gas expansion

I'm slightly confused by a common situation in thermochemistry. 


Consider a container with gas inside and a movable, massless piston. The pressure of the gas inside is 6 atm and the external pressure is 5 atm. Thus the gas will expand. However since the piston is massless, the two forces on the piston must be equal by Newton's 2nd Law. But the pressure from the gas and the pressure from the outside atmosphere are clearly unequal, so what's the problem here?


