diff --git a/mrbgems/mruby-bigint/core/bigint.c b/mrbgems/mruby-bigint/core/bigint.c
index 549b9df19..5c64c7d87 100644
--- a/mrbgems/mruby-bigint/core/bigint.c
+++ b/mrbgems/mruby-bigint/core/bigint.c
@@ -373,6 +373,8 @@ trim(mpz_t *x)
   while (x->sz && x->p[x->sz-1] == 0) {
     x->sz--;
   }
+  /* Maintain invariant: sz == 0 implies sn == 0 (zero is canonical). */
+  if (x->sz == 0) x->sn = 0;
 }
 
 /* z = x + y, without regard for sign */