diff --git a/src/printers/marabou.ml b/src/printers/marabou.ml
index 71eb46a980929697e75c3537a36b9a22991f92b8..72568d3befd211957ccbf1effbc7de9cbe0a999d 100644
--- a/src/printers/marabou.ml
+++ b/src/printers/marabou.ml
@@ -21,7 +21,8 @@ let number_format =
     Why3.Number.hex_int_support = `Unsupported;
     Why3.Number.oct_int_support = `Unsupported;
     Why3.Number.bin_int_support = `Unsupported;
-    Why3.Number.negative_real_support = `Default;
+    Why3.Number.negative_real_support =
+      `Custom (fun fmt f -> Fmt.pf fmt "-%t" f);
     Why3.Number.dec_real_support = `Default;
     Why3.Number.hex_real_support = `Unsupported;
     Why3.Number.frac_real_support = `Unsupported (fun _ _ -> assert false);