สอบถามการ join multiple datatable ใน vb.net แล้วมานเกิดปัญหา

Dim joinedResult = From a In dt.AsEnumerable _
                                Join b In dtAR.AsEnumerable() On b.Field(Of String)("DATE") Equals a.Field(Of String)("Settlement") _
                                Join c In dtPort.AsEnumerable() On c.Field(Of String)("VALUEDATE") Equals a.Field(Of String)("Settlement") _
                            Select Settlement = a.Field(Of String)("Settlement"), _
                                    Balance = a.Field(Of String)("Balance"), _
                                    Change = a.Field(Of String)("Change"), _
                                    Benefit = a.Field(Of String)("Benefit"), _
                                    Balance1 = a.Field(Of String)("Balance1"), _
                                    AR = a.Field(Of String)("AR")