i need to get the guid of anther entity

Private Function FindBranch(ByVal Name As String) As String
FindBranch = ""
Dim CS As New ServiceReference1.ColumnSet
CS.Columns = {"branchid", "name"}
Dim QE As New ServiceReference1.QueryExpression
QE.ColumnSet = CS
QE.EntityName = "mbmhr_branch"

Dim CE As New ServiceReference1.ConditionExpression
CE.AttributeName = "name"
CE.Operator = ServiceReference1.ConditionOperator.Equal
CE.Values = New Object() {Name}
Dim FE As New ServiceReference1.FilterExpression
FE.FilterOperator = ServiceReference1.LogicalOperator.And
FE.Conditions = New ServiceReference1.ConditionExpression() {CE}
QE.Criteria = FE

Dim EC As ServiceReference1.EntityCollection
Dim OC As New ServiceReference1.OrganizationServiceClient
EC = OC.RetrieveMultiple(QE)
For i As Integer = 0 To EC.Entities.Count - 1
FindBranch = EC.Entities(i).Attributes(0).Value.ToString
Next
End Function

i get a error invalid column name