A surgical stapling instrument comprising an end effector is disclosed. The end effector comprises an anvil and a cartridge jaw comprising a channel, wherein the channel comprises a lockout recess defined therein. A staple cartridge is replaceably seated in the channel. A firing member is configured to distally advance a sled during a staple firing stroke. The firing member comprises a first portion configured to engage the anvil, a second portion configured to engage the cartridge jaw, and a lockout portion positioned between the first portion and the second portion. The lockout portion is configured to engage the lockout recess when the sled is not in a proximal position prior to the staple firing stroke. The surgical stapling instrument is unable to perform the staple firing stroke when the lockout portion engages the lockout recess.