Yacc NULL in OCaml?

I am implementing the following grammar in OCamlyacc and OCamllex:

The OCaml type declaration for my IF-ELSE clauses is as such:

(* Some code not shown *)
and stmt  = ASSIGN of lv * exp
      | IF of exp * stmt * stmt 
      | WHILE of exp * stmt
      | DOWHILE of stmt * exp
      | READ of id
      | PRINT of exp 
      | BLOCK of block
(* Some code not shown *)

I can define the IF-ELSE portion in OCamlyacc as such:

stmt:
   |    IF LPAREN e RPAREN stmt                 { S.IF ($3, $5, ???) } /*line 1*/
   |    IF LPAREN e RPAREN stmt ELSE stmt       { S.IF ($3, $5, $7) }  /*line 2*/

But, how can I put "NULL" for a statement type ("stmt"), where the question marks are for the IF-Statement that does not have an ELSE (line 1)? I do not have access to a statement type that only has an expression ("exp") and only one statement.

I thought about putting a "while(0){print(0)}" statement there, but that is not how you are suppose to do it, especially because it will parse in a while-statement when it shouldn't.