Getting an error when scheduling an Alert

Customer is modifying a custom Alert based from EMPS API to Workers REST API with Assignment DFF attribute in the filters.

When same Alert was running for years now based on EMPS API, no error had popped up along with the same Assignment DFF filter.

When the Alert is executed for Workers REST API getting the below error. What could be the reason?

Error in ${AlertUtils.toString(AlertUtils.empManagerList(AlertUtils.userName(WorkListUser).PersonId).UserName)} Execution No signature of method: oracle.apps.hcm.common.core.alerts.processing.AlertFunctionImpl.userName() is applicable for argument types: (org.codehaus.groovy.runtime.NullObject) values: [null] Possible solutions: userName(java.util.List), userName(java.lang.String)