how to invoke wcf service having list parameter

I have a webservice as below

<OperationContract()><Web.WebGet(UriTemplate:="/GetData?strErrMsg={strErrMsg}&chrErrFlg={chrErrFlg}", ResponseFormat:=WebMessageFormat.Json,BodyStyle:=WebMessageBodyStyle.Wrapped)> _
    Function GetData(ByRef strErrMsg As System.Collections.Generic.List(Of String), ByRef chrErrFlg As String) As String

And I am trying to fetch it as

http:localhost//BedTypeMasterService.svc/GetData?strErrMsg="ff"&chrErrFlg="dd"

But it is giving following error

Operation 'GetData' in contract 'iBedTypeMaster' has a query variable named 'strErrMsg' of type 'System.Collections.Generic.List1[System.String]', but type 'System.Collections.Generic.List1[System.String]' is not convertible by 'QueryStringConverter'. Variables for UriTemplate query values must have types that can be converted by 'QueryStringConverter'.

Please help me out to solve this