รบกวนช่วยดูปัญหาด้วยค่ะ Conversion from string to type 'Double' is not valid

Sub CalculateInterest()
        Dim month, b_month, year As String
        Dim rate, b_rate, interest, b_interest, savingdep, plus As Double
        Dim i As Integer
        Dim id As Integer


        '=========================================================================================='
        strsql = " SELECT  * FROM EMPLOYEEDT INNER JOIN EMPLOYEE ON EMPLOYEE.MEMP = EMPLOYEEDT.MEMP WHERE EMPLOYEE.STATUS = 'YES'"
        dtAdapter = New SqlDataAdapter(strsql, myConn)
        dtAdapter.Fill(dt3)

        For i = 0 To dt3.Rows.Count - 1
            id = dt3.Rows(i)("MEMP")
            savingdep = dt3.Rows(i)("SAVINGDEP")


            '================================================================================
            b_month = SplitB_Month(dateInput.Text)

            Select Case b_month
                Case "มกราคม"
                    b_rate = 0.666667
                Case "กุมภาพันธ์"
                    b_rate = 1.333333
                Case "มีนาคม"
                    b_rate = 2.0
                Case "เมษายน"
                    b_rate = 2.666667
                Case "พฤษภาคม"
                    b_rate = 3.333333
                Case "มิถุนายน"
                    b_rate = 4.0
                Case "กรกฎาคม"
                    b_rate = 4.666667
                Case "สิงหาคม"
                    b_rate = 5.333333
                Case "กันยายน"
                    b_rate = 6.0
                Case "ตุลาคม"
                    b_rate = 6.666667
                Case "พฤศจิกายน"
                    b_rate = 7.333333
                Case "ธันวาคม"
                    b_rate = 8.0
            End Select
            b_interest = savingdep * b_rate / 100

            '========================================================================================='
            month = SplitMonth(dateInput.Text)
            Select Case month
                Case "มกราคม"
                    rate = 0.666667
                Case "กุมภาพันธ์"
                    rate = 1.333333
                Case "มีนาคม"
                    rate = 2.0
                Case "เมษายน"
                    rate = 2.666667
                Case "พฤษภาคม"
                    rate = 3.333333
                Case "มิถุนายน"
                    rate = 4.0
                Case "กรกฎาคม"
                    rate = 4.666667
                Case "สิงหาคม"
                    rate = 5.333333
                Case "กันยายน"
                    rate = 6.0
                Case "ตุลาคม"
                    rate = 6.666667
                Case "พฤศจิกายน"
                    rate = 7.333333
                Case "ธันวาคม"
                    rate = 8.0
            End Select
            interest = savingdep * rate / 100

            '=============================================================================================='
            plus = interest - b_interest
            plus = Math.Round(plus, 2)

            '==============================================================================================='
            strsql = " UPDATE EMPLOYEEDT SET AINTEREST = AINTEREST + '" + plus + "'  WHERE MEMP = '" + id + "' " 'ERROR ที่บรรทัดนี้ค่ะ
            cmd = New SqlCommand(strsql, myConn)
            cmd.ExecuteNonQuery()
        Next

    End Sub