Разрешить сохранение нулевых значений в базе данных

Я использую Visual Studio .NET 2003, кнопка добавления работает нормально, когда все текстовые поля, поля со списком заполнены данными, но при тестировании, не заполняя поля данными, оставляя его NULL, он возвращает ошибку, говорящую: «Строка не была распознана как допустимый DateTime»

У меня есть текстовое поле с именем txtPurchasedDate.

Моя хранимая процедура

 CREATE PROCEDURE AddOfficeEquipmentProfile
    (
    @OE_ID      varchar(11)     =   NULL,
    @OE_Category        char(3)         =   NULL,
    @OE_SubCategory char(3)         =   NULL,
    @OE_Name        varchar(35)     =   NULL,
    @OE_User        varchar(35)     =   NULL,
    @OE_Brand       varchar(15)     =   NULL,
    @OE_Model       varchar(35)     =   NULL,
    @OE_Specs       varchar(1000)       =   NULL,
    @OE_SerialNo        varchar(35)     =   NULL,
    @OE_PropertyNo  varchar(35)     =   NULL,
    @OE_MacAddress  varchar(100)        =   NULL,   
    @OE_Static_IP       varchar(15)     =   NULL,
    @OE_Vendor      varchar(35)     =   NULL,
    @OE_PurchaseDate    smalldatetime       =   NULL,
    @OE_WarrantyInclusiveYear   int     =   NULL,
    @OE_WarrantyStatus  char(2)         =   NULL,
    @OE_Status      varchar(15)     =   NULL,
    @OE_Dept_Code   char(3)         =   NULL,
    @OE_Location_Code   char(8)         =   NULL,
    @OE_Remarks     varchar(1000)       =   NULL
    )
    AS

    INSERT INTO tblOfficeEquipmentProfile (OE_ID, OE_Category, OE_SubCategory, OE_Name, OE_User, OE_Brand, OE_Model, OE_Specs, OE_SerialNo,
    OE_PropertyNo, OE_MacAddress, OE_Static_IP, OE_Vendor, OE_PurchaseDate, OE_WarrantyInclusiveYear, OE_WarrantyStatus, OE_Status, OE_Dept_Code,
    OE_Location_Code, OE_Remarks ) 
    VALUES (@OE_ID, @OE_Category, @OE_SubCategory, @OE_Name, @OE_User, @OE_Brand, @OE_Model, 
    @OE_Specs, @OE_SerialNo, @OE_PropertyNo, @OE_MacAddress, @OE_Static_IP, @OE_Vendor, @OE_PurchaseDate, @OE_WarrantyInclusiveYear, @OE_WarrantyStatus,
    @OE_Status, @OE_Dept_Code, @OE_Location_Code, @OE_Remarks)

    IF @@ERROR<>0
        BEGIN
            ROLLBACK TRANSACTION
            RETURN 0
        END 
    ELSE
        BEGIN
            COMMIT TRANSACTION
            RETURN 1
        END
    GO

Мой код кнопки добавления Vb.net

 Dim cmd As SqlCommand = sqlconn.CreateCommand
        sqlconn.Open()
        cmd.CommandType = CommandType.StoredProcedure
        cmd.CommandText = "AddOfficeEquipmentProfile"

    cmd.Parameters.Add("@OE_ID", SqlDbType.VarChar, 11, "oeq-su-999")
    cmd.Parameters.Add("@OE_Category", SqlDbType.Char, 3, "COM")
    cmd.Parameters.Add("@OE_SubCategory", SqlDbType.Char, 3, "SU")
    cmd.Parameters.Add("@OE_Name", SqlDbType.VarChar, 35, "adminpmis01")
    cmd.Parameters.Add("@OE_User", SqlDbType.VarChar, 35, "Ivan")
    cmd.Parameters.Add("@OE_Brand", SqlDbType.VarChar, 15, "DELL")
    cmd.Parameters.Add("@OE_Model", SqlDbType.VarChar, 35, "optiplex")
    cmd.Parameters.Add("@OE_Specs", SqlDbType.VarChar, 1000, "dualcore")
    cmd.Parameters.Add("@OE_SerialNo", SqlDbType.VarChar, 35, "sgh5960")
    cmd.Parameters.Add("@OE_PropertyNo", SqlDbType.VarChar, 35, "j7h7h6g6f2")
    cmd.Parameters.Add("@OE_MacAddress", SqlDbType.VarChar, 100, "j7h7:h6g6f2")
    cmd.Parameters.Add("@OE_Static_IP", SqlDbType.VarChar, 15, "192.168.1.5")
    cmd.Parameters.Add("@OE_Vendor", SqlDbType.VarChar, 35, "ADWAYS")

    cmd.Parameters.Add("@OE_PurchaseDate", SqlDbType.SmallDateTime)
    cmd.Parameters.Add("@OE_WarrantyInclusiveYear", SqlDbType.Int)
    cmd.Parameters.Add("@OE_WarrantyStatus", SqlDbType.Char, 2, "IN")
    cmd.Parameters.Add("@OE_Status", SqlDbType.VarChar, 15, "Good")
    cmd.Parameters.Add("@OE_Dept_Code", SqlDbType.Char, 3, "ADM")
    cmd.Parameters.Add("@OE_Location_Code", SqlDbType.Char, 8, "ADM_OFC")
    cmd.Parameters.Add("@OE_Remarks", SqlDbType.VarChar, 1000, "ACTIVE")
    cmd.Parameters("@OE_ID").Value = txtOEID.Text
    cmd.Parameters("@OE_Category").Value = cmbCategory.Text
    cmd.Parameters("@OE_SubCategory").Value = cmbSubCategory.Text
    cmd.Parameters("@OE_Name").Value = txtName.Text
    cmd.Parameters("@OE_User").Value = txtUser.Text
    cmd.Parameters("@OE_Brand").Value = cmbBrand.Text
    cmd.Parameters("@OE_Model").Value = cmbModel.Text
    cmd.Parameters("@OE_Specs").Value = txtSpecs.Text
    cmd.Parameters("@OE_SerialNo").Value = txtSerialNo.Text
    cmd.Parameters("@OE_PropertyNo").Value = txtPropertyNo.Text
    cmd.Parameters("@OE_MacAddress").Value = txtMacAddress.Text
    cmd.Parameters("@OE_Static_IP").Value = txtStaticIp.Text
    cmd.Parameters("@OE_Vendor").Value = txtVendor.Text
    cmd.Parameters("@OE_PurchaseDate").Value = txtPurchaseDate.Text
    cmd.Parameters("@OE_WarrantyInclusiveYear").Value = txtWarrantyInclusiveYear.Text
    cmd.Parameters("@OE_WarrantyStatus").Value = txtWarrantyStatus.Text
    cmd.Parameters("@OE_Status").Value = txtStatus.Text
    cmd.Parameters("@OE_Dept_Code").Value = cmbDeptCode.Text
    cmd.Parameters("@OE_Location_Code").Value = cmbLocationCode.Text
    cmd.Parameters("@OE_Remarks").Value = txtRemarks.Text
    cmd.ExecuteNonQuery()
    MsgBox("Successfully Added Equipment Profile")
    sqlconn.Close()