diff --git a/sexp.c b/sexp.c
index 5b355312..1bdd9c24 100644
--- a/sexp.c
+++ b/sexp.c
@@ -2192,6 +2192,11 @@ sexp sexp_write_one (sexp ctx, sexp obj, sexp out, sexp_sint_t bound) {
       sexp_write_string(ctx, "#<procedure ", out);
       x = sexp_bytecode_name(sexp_procedure_code(obj));
       sexp_write_one(ctx, sexp_synclop(x) ? sexp_synclo_expr(x): x, out, bound+1);
+      sexp_write_string(ctx, " ", out);
+      sexp_write_one(ctx, sexp_make_fixnum(sexp_procedure_num_args(obj)), out, bound+1);
+      if (sexp_procedure_variadic_p(obj)) {
+        sexp_write_string(ctx, "+", out);
+      }
 #if SEXP_USE_DEBUG_VM
       if (sexp_procedure_source(obj)) {
         sexp_write_string(ctx, " ", out);