The type 'VarString' is not supported as a return value. with RDP parmQuery method

Hi All,

My RDP was working before add parmquery method:

this is class declaration method of my  data contract:

class ETGExcDiffRepContract implements SysOperationValidatable
{
    StartDate           startDate;
    EndDate             endDate;
    str                 packedQuery;
}

this is my parmquery method:

[DataMemberAttribute,
    AifQueryTypeAttribute('_packedQuery', querystr(GeneralJournalEntryQuery_TR))
]
public str parmQuery(str _packedQuery = packedQuery)
{
    packedQuery = _packedQuery;
    return packedQuery;
}

when I run my controller class I get "The type 'VarString' is not supported as a return value" error. What should I do?