Error: Smoke Testing Docker Agent with Jenkins

Dears:
In video: Preparing Jenkins for Docker Agent-Based Builds
In step: Smoke Testing Docker Agent with Jenkins

When running the pipeline, I received below error:

Started by user CI Admin
org.codehaus.groovy.control.MultipleCompilationErrorsException: startup failed:
WorkflowScript: 3: Invalid agent type "docker" specified. Must be one of [any, label, none] @ line 3, column 9.
           docker { image 'node:7-alpine' }
           ^

I solved it by following steps in this https://theserverside.com/blog/Coffee-Talk-Java-News-Stories-and-Opinions/Fix-Jenkins-Invalid-agent-type-Docker-specified-any-label-none-error

and restarted Jenkins.