Help on Run - time error 91

[nuckles]

Hi guys. I just wanted to ask you help in this tricky little thingy. I'm trying to create a log in action which takes the username and password from an access .mdb file and verifies it with the ones entered in a text box. but somehow I seem to be getting a Run - time error 91 (Object variable or With block variable not set)

The code is as below and the problematic line of coding has been highlighted.

Private Sub login_btn_Click()
Set rs = db.OpenRecordset("Select emp_username from employee where emp_username = '" & username_txt.Text & "'")
If Not rs.RecordCount = 0 Then
    Set rs = db.OpenRecordset("Select emp_pass from employee where emp_pass = '" & password_txt.Text & "'")
    If Not rs.RecordCount = 0 Then
        login_frame.Visible = False 'hide login frame
        welcome_frame.Visible = True 'unhide welcome frame
        btn_home.Visible = False 'unhide buttons
        btn_rent_car.Visible = False
        btn_customer.Visible = False
        btn_car.Visible = False
        btn_account.Visible = False
        btn_employee.Visible = False
        
        
    Else
        MsgBox "Password incorrect"
        password_txt.SetFocus
    End If
Else
    MsgBox "Username incorrect"
    username_txt.SetFocus
End If

End Sub

I have added a reference to the ADO and also created a module which goes like this

Public rs As DAO.Recordset
Public db As DAO.Database

Public Sub Main()
Set db = OpenDatabase(App.Path + "/EasyRent.mdb")
main_form.Show
End Sub

I'm still doing my diploma so any help at all would be greatly appreciated. Thanking you guys i advance.