How to create custom sequence generator for below it is throwing error Unknown integral data type for ids : java.lang.String

I am getting error while creating auto generated key for a string primary key error: org.hibernate.id.IdentifierGenerationException: Unknown integral data type for ids : java.lang.String HBM.XML below

<class name="CmRetroEvt" table="CM_RETRO_EVT" schema="CISADM" >
    
        <id name="ext_source_id" type="string" column="EXT_SOURCE_ID">
            <generator class="sequence">
            <param name="sequence_name">EXT_SOURCE_ID_SEQ_NBR</param>
            </generator>
        </id>

Got the following error any plz suggest what needs to be done to overcome this issue :-

org.hibernate.id.IdentifierGenerationException: Unknown integral data type for ids : java.lang.String