diff --git a/src/printers/vnnlib.ml b/src/printers/vnnlib.ml
index fe451527ff2cc1b6becc37497b9a7e329d400686..29758ee6ce16a920db6930c6c3e6fd5092e28258 100644
--- a/src/printers/vnnlib.ml
+++ b/src/printers/vnnlib.ml
@@ -435,6 +435,7 @@ let print_goal info fmt (pr, t) =
 
 let rec negate_term info t =
   match t.Term.t_node with
+  | Tnot t -> t
   | Tbinop (Tand, t1, t2) ->
     Term.t_or (negate_term info t1) (negate_term info t2)
   | Tbinop (Tor, t1, t2) ->