Trying to disallow duplicate entries in a text fiel on a form

I'm using this code in an attempt to disallow duplicate entries in a form field.  But I get an error...  "Data type mismatch in criteria expression".  Note:  Serial is a text field in the table.

Private Sub txtSerial_BeforeUpdate(Cancel As Integer)

    Dim strCriteria As String

    strCriteria = "Serial = " & Me.txtSerial & ""

    If DCount("Serial", "tblEquipment", strCriteria) > 0 Then
        Me.Undo
        MsgBox "Warning: Serial " & txtSerial & " already exists." & vbCr & vbCr & "You cannot enter duplicate serial numbers.", vbInformation, "Duplicate Information"
        Cancel = True
    End If

End Sub

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