I need help verifying a variable on creation of Sales Invoice

I there,

   I'm struggling to get a verification if a Variable is empty or not on creation of a sales order. 

   The Variable is in each line of the sales order, i need to emmit a warning and block te creation if one of the lines has the var empty 

   Here is my code:

Case ACTION
  When "VERIF_CRE"

        For I = 0 To [M:SIH4]NBLIG
         If [F:SIH4]ZSERIE = "" or [F:SIH4]ZSERIE = "0"
            [L]SERIE = 1
         Endif
        Next I
    
        If [L]SERIE = 1 :
            GERR=1 : GMESSAGE= "teste" : OK=0 :
            Return
        Endif
Endcase
Endif        


any help will be appreciated.