Invalid custom history level error on restart

Hello, I am trying to implement custom history level, I have implemented custom history level plugin. But when I change the level in application yaml as follows
history-level: custom-history-level and restart engine, I am getting the following error:

Caused by: org.camunda.bpm.engine.ProcessEngineException: invalid history level: custom-history-level
at org.camunda.bpm.engine.impl.cfg.ProcessEngineConfigurationImpl.initHistoryLevel(ProcessEngineConfigurationImpl.java:1970)
at org.camunda.bpm.engine.impl.cfg.ProcessEngineConfigurationImpl.init(ProcessEngineConfigurationImpl.java:808)
at org.camunda.bpm.engine.impl.cfg.ProcessEngineConfigurationImpl.buildProcessEngine(ProcessEngineConfigurationImpl.java:797)
at org.camunda.bpm.engine.spring.SpringTransactionsProcessEngineConfiguration.buildProcessEngine(SpringTransactionsProcessEngineConfiguration.java:63)
at org.camunda.bpm.engine.spring.ProcessEngineFactoryBean.getObject(ProcessEngineFactoryBean.java:55)
at org.camunda.bpm.engine.spring.ProcessEngineFactoryBean.getObject(ProcessEngineFactoryBean.java:34)
at org.springframework.beans.factory.support.FactoryBeanRegistrySupport.doGetObjectFromFactoryBean(FactoryBeanRegistrySupport.java:171)

Can you please let me know, is there anything else to be done. I have followed example from here : https://github.com/camunda/camunda-bpm-examples/tree/master/process-engine-plugin/custom-history-level