How Can I Valid A Compound Key With Dlookup Function

In a form named PRODUCTION there are 3 text box for invoice data: xtype, xserie, xnumber

I need valid this data from the INVOICE table, where its key is a compound key (TPINV+SERINV+NUMINV)

I tried with:

Private Sub xnumber_AfterUpdate()
Dim searchinv as string
searchinv=Dlookup("*","[Invoice]", [tpinv]="& Forms![production].[xtype] and [xserie]= &Forms![production].[xnumber] and "&Forms![xnumber]
If searchinv ="" then
MsgBox ("Invoice doesn't exist'")
Cancel = True
End If
End Sub

... but it doesnt work.