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Private Sub Command4_Click()
Dim vPass As String
 If IsNull([usernametxt]) = True Then
        MsgBox "Username is required", vbOKOnly, "Required Data"
        
    ElseIf IsNull([answertxt]) = True Then
        MsgBox "Password is required", vbOKOnly, "Required Data"

End If
' Evaluate filter before it is passed to DLookup function.
     strFilter = "[USERNAME]= '" & Me.usernametxt & "' And [ANSWER]= '" & Me.answertxt & "'"

    ' Look up product's unit price and assign it to the UnitPrice control.
     vPass = DLookup("PASSWORD", "SYS_USER", strFilter)

If IsNull(vPass) = False Then
Me.passwordtxt.Value = vPass
Else
MsgBox "You have entered an incorrect username or answer."
End If
End Sub

i use this code to run a forget password in a login system.
it works just fine, but have one error if u entered wrong information it gives me invalid use of null and points at(vPass = DLookup("PASSWORD", "SYS_USER", strFilter).

Comments
again started anew thread for the same old question.

Your variable "vPass" should be type "variant".

You may also wish to exit the sub if you have invalid entries, like so:

Private Sub Command4_Click()
Dim vPass As Variant           '<------------------------ Should be variant, not string datatype
If IsNull([usernametxt]) = True Then
    MsgBox "Username is required", vbOKOnly, "Required Data"
    Exit Sub                   '<------------------------ Leave Subroutine on invalid username
ElseIf IsNull([answertxt]) = True Then
    MsgBox "Password is required", vbOKOnly, "Required Data"
    Exit Sub                   '<------------------------ Leave Subroutine on invalid answer text
End If
' Evaluate filter before it is passed to DLookup function.
strFilter = "[USERNAME]= '" & Me.usernametxt & "' And [ANSWER]= '" & Me.answertxt & "'"

' Look up product's unit price and assign it to the UnitPrice control.
vPass = DLookup("PASSWORD", "SYS_USER", strFilter)

If IsNull(vPass) = False Then
    Me.passwordtxt.Value = vPass
Else
    MsgBox "You have entered an incorrect username or answer."
End If

End Sub

Hope this helps.

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