A cap for a fluid line connector having a male luer lock fitting , the cap having a first side and a second side opposite the first side , the cap comprising: a barrier defining a keyway , wherein the keyway is centred on a longitudinal axis , the barrier comprising two or more circumferentially-spaced barrier portions, including a first barrier portion and a second barrier portion , to define the keyway , each circumferential spacing defining a slot configured to receive a correspondingly-shaped lug of a correspondingly-shaped female luer lock fitting to permit the correspondingly-shaped female luer lock fitting to pass therethrough, each barrier portion having a first sidewall configured to prevent rotation of the correspondingly-shaped female luer lock fitting in a first circumferential direction when a lug of the correspondingly-shaped female luer lock fitting is circumferentially aligned with the first sidewall ; characterised in that the cap further comprises a stopper extending axially from the second barrier portion towards the first side of the cap , wherein the stopper has a stopper wall that is coplanar with the first sidewall of the second barrier portion ; wherein the shape of the keyway is configured to permit the correspondingly-shaped female luer lock fitting to pass axially therethrough from the first side to engage the male luer lock fitting of the fluid line connector; and wherein the stopper is configured to prevent rotation of the correspondingly-shaped female luer lock fitting in the first circumferential direction when a lug of the correspondingly-shaped female luer lock fitting is circumferentially aligned with the stopper . a barrier defining a keyway , wherein the keyway is centred on a longitudinal axis , the barrier comprising two or more circumferentially-spaced barrier portions, including a first barrier portion and a second barrier portion , to define the keyway , each circumferential spacing defining a slot configured to receive a correspondingly-shaped lug of a correspondingly-shaped female luer lock fitting to permit the correspondingly-shaped female luer lock fitting to pass therethrough, each barrier portion having a first sidewall configured to prevent rotation of the correspondingly-shaped female luer lock fitting in a first circumferential direction when a lug of the correspondingly-shaped female luer lock fitting is circumferentially aligned with the first sidewall ; characterised in that the cap further comprises a stopper extending axially from the second barrier portion towards the first side of the cap , wherein the stopper has a stopper wall that is coplanar with the first sidewall of the second barrier portion ; wherein the shape of the keyway is configured to permit the correspondingly-shaped female luer lock fitting to pass axially therethrough from the first side to engage the male luer lock fitting of the fluid line connector; and wherein the stopper is configured to prevent rotation of the correspondingly-shaped female luer lock fitting in the first circumferential direction when a lug of the correspondingly-shaped female luer lock fitting is circumferentially aligned with the stopper . <EOS>