System.Data.OleDb.OleDbException: Data type mismatch in criteria expression.

Hwy this code just don't work : I keep getting error of ( System.Data.OleDb.OleDbException: Data type mismatch in criteria expression.):

    Function UpdateCustomerDetail(ByVal userID As String, ByVal first As String, ByVal surname As String, ByVal birthdate As Date, ByVal address As String, ByVal postCode As String, ByVal phone As String, ByVal email As String) As Integer
        Dim connectionString As String = "Provider=Microsoft.Jet.OLEDB.4.0; Ole DB Services=-4; Data Source=C:\Documents an"& _
"d Settings\Administrator\My Documents\ECommerceAssignment\ASPCode\JEANS.mdb"
        Dim dbConnection As System.Data.IDbConnection = New System.Data.OleDb.OleDbConnection(connectionString)

        Dim queryString As String = "UPDATE [customer] SET [First]=@First, [Surname]=@Surname, [Birthdate]=@Birthdate,"& _
" [Address]=@Address, [PostCode]=@PostCode, [Phone]=@Phone, [Email]=@Email WHERE "& _
"([customer].[userID] = @userID)"
        Dim dbCommand As System.Data.IDbCommand = New System.Data.OleDb.OleDbCommand
        dbCommand.CommandText = queryString
        dbCommand.Connection = dbConnection

        Dim dbParam_userID As System.Data.IDataParameter = New System.Data.OleDb.OleDbParameter
        dbParam_userID.ParameterName = "@userID"
        dbParam_userID.Value = userID
        dbParam_userID.DbType = System.Data.DbType.String
        dbCommand.Parameters.Add(dbParam_userID)
        Dim dbParam_first As System.Data.IDataParameter = New System.Data.OleDb.OleDbParameter
        dbParam_first.ParameterName = "@First"
        dbParam_first.Value = first
        dbParam_first.DbType = System.Data.DbType.String
        dbCommand.Parameters.Add(dbParam_first)
        Dim dbParam_surname As System.Data.IDataParameter = New System.Data.OleDb.OleDbParameter
        dbParam_surname.ParameterName = "@Surname"
        dbParam_surname.Value = surname
        dbParam_surname.DbType = System.Data.DbType.String
        dbCommand.Parameters.Add(dbParam_surname)
        Dim dbParam_birthdate As System.Data.IDataParameter = New System.Data.OleDb.OleDbParameter
        dbParam_birthdate.ParameterName = "@Birthdate"
        dbParam_birthdate.Value = birthdate
        dbParam_birthdate.DbType = System.Data.DbType.DateTime
        dbCommand.Parameters.Add(dbParam_birthdate)
        Dim dbParam_address As System.Data.IDataParameter = New System.Data.OleDb.OleDbParameter
        dbParam_address.ParameterName = "@Address"
        dbParam_address.Value = address
        dbParam_address.DbType = System.Data.DbType.String
        dbCommand.Parameters.Add(dbParam_address)
        Dim dbParam_postCode As System.Data.IDataParameter = New System.Data.OleDb.OleDbParameter
        dbParam_postCode.ParameterName = "@PostCode"
        dbParam_postCode.Value = postCode
        dbParam_postCode.DbType = System.Data.DbType.String
        dbCommand.Parameters.Add(dbParam_postCode)
        Dim dbParam_phone As System.Data.IDataParameter = New System.Data.OleDb.OleDbParameter
        dbParam_phone.ParameterName = "@Phone"
        dbParam_phone.Value = phone
        dbParam_phone.DbType = System.Data.DbType.String
        dbCommand.Parameters.Add(dbParam_phone)
        Dim dbParam_email As System.Data.IDataParameter = New System.Data.OleDb.OleDbParameter
        dbParam_email.ParameterName = "@Email"
        dbParam_email.Value = email
        dbParam_email.DbType = System.Data.DbType.String
        dbCommand.Parameters.Add(dbParam_email)

        Dim rowsAffected As Integer = 0
        dbConnection.Open
        Try
            rowsAffected = dbCommand.ExecuteNonQuery
        Finally
            dbConnection.Close
        End Try

        Return rowsAffected
    End Function

Sub Button2_Click(sender As Object, e As EventArgs)
Dim fname as String,sname as String,add as String,pcode as String,phone as String,email as String
Dim bdate as Date
Dim userid as String

userid = Label2.text

fname = TextBox1.text
sname = TextBox2.text
bdate = TextBox3.text
add = TextBox4.text
pcode = TextBox5.text
phone = TextBox6.text
email = TextBox7.text
'response.write(userid & "," & fname & ",")
UpdateCustomerDetail(userid,fname,sname,bdate,add,pcode,phone,email)

End Sub