Lab8: invalid invocation: chaincode 'sacc' is already initialized but called as init

At the last bit of Lab 8 there's a task to try out and init a chaincode when invoked. Eveything seems to work fine except the last step:

peer chaincode invoke -o orderer.example.com:7050 --tls --cafile $ORDERER_TLS_CA -C allarewelcome -n sacc --peerAddresses $CORE_PEER_ADDRESS --tlsRootCertFiles $CORE_PEER_TLS_ROOTCERT_FILE -c '{"function":"set","args":["key4", "44444444"]}' --waitForEvent --isInit

fails with:

Error: endorsement failure during invoke. response: status:500 message:"error in simulation: failed to execute transaction 1c647e3d3c24e4db2e111d03b8eb34b4a6a5ec0c493c6d9ef7586af3737e4e43: invalid invocation: chaincode 'sacc' is already initialized but called as init" 

However, it works just fine if I remove --isInit from the invoke command. Is it possible that --isInit is no longer required?