diff --git a/mrbgems/mruby-bigint/core/bigint.c b/mrbgems/mruby-bigint/core/bigint.c
index bb5fc918f..331c30169 100644
--- a/mrbgems/mruby-bigint/core/bigint.c
+++ b/mrbgems/mruby-bigint/core/bigint.c
@@ -354,10 +354,11 @@ uadd(mpz_t *z, mpz_t *x, mpz_t *y)
   /* Core multi-limb addition with carry propagation */
   mp_dbl_limb c = 0;
   size_t i;
+  size_t min_sz = (x->sz < y->sz) ? x->sz : y->sz;
 
   /* Add overlapping limbs from both operands */
   /* 4x unrolled loop for better performance */
-  for (i = 0; i + 4 <= x->sz; i += 4) {
+  for (i = 0; i + 4 <= min_sz; i += 4) {
     c += (mp_dbl_limb)y->p[i] + (mp_dbl_limb)x->p[i];
     z->p[i] = LOW(c);
     c >>= DIG_SIZE;
@@ -375,13 +376,20 @@ uadd(mpz_t *z, mpz_t *x, mpz_t *y)
     c >>= DIG_SIZE;
   }
 
-  /* Handle remaining elements */
-  for (; i < x->sz; i++) {
+  /* Handle remaining elements in overlap */
+  for (; i < min_sz; i++) {
     c += (mp_dbl_limb)y->p[i] + (mp_dbl_limb)x->p[i];
     z->p[i] = LOW(c);
     c >>= DIG_SIZE;
   }
 
+  /* Add remaining limbs from x if it's larger */
+  for (; i < x->sz; i++) {
+    c += x->p[i];
+    z->p[i] = LOW(c);
+    c >>= DIG_SIZE;
+  }
+
   /* Add remaining limbs from larger operand */
   /* 4x unrolled loop for better performance */
   for (; i + 4 <= y->sz; i += 4) {