You have to use the IsNull() function:
Code:
Dim ValoreEsistente As String
Dim MioConteggio As String
ValoreEsistente = Me.Voto_04
MioConteggio = Left(ValoreEsistente, 3) ' see below
If MioConteggio = "04 -" Or IsNull(ValoreEsistente) Then
Exit Sub
Else
Me.[Voto_04] = "04 -" & ValoreEsistente
End If
-------------------------
Other items:
Don't have to type ".Value" ...... VALUE is the default property.
If there is a space in the string "04 -", the length is 4, not 3. (zero, four, space,dash)
The line
[Voto_04]. Value = "04 -" & Me [Voto_04] - is missing a dot between "ME" and "[Voto_04]"
The variable "ValoreEsistente" is declared as a Variant, "MioConteggio" is declared as a String.