A have test kubernetes on-premise cluster on centos 7.4. Current kubernetes version is 1.10.4. I am trying to upgrade to 1.11.5 using kubespray

The command is:

ansible-playbook upgrade-cluster.yml  -b -i inventory/k8s-test/hosts.ini -e kube_version=v1.11.5

Masters are upgraded successfully, but nodes are not. The error is:

fatal: [kubernodetst1]: FAILED! => {"changed": true, "cmd": ["/usr/local/bin/kubeadm", "join", "--config", "/etc/kubernetes/kubeadm-client.conf", "--ignore-preflight-errors=all", "--discovery-token-unsafe-skip-ca-verification"], "delta": "0:00:00.040038", "end": "2018-12-13 15:55:56.162387", "msg": "non-zero return code", "rc": 3, "start": "2018-12-13 15:55:56.122349", "stderr": "discovery: Invalid value: \"\": using token-based discovery without discoveryTokenCACertHashes can be unsafe. set --discovery-token-unsafe-skip-ca-verification to continue", "stderr_lines": ["discovery: Invalid value: \"\": using token-based discovery without discoveryTokenCACertHashes can be unsafe. set --discovery-token-unsafe-skip-ca-verification to continue"], "stdout": "", "stdout_lines": []}