diff --git a/src/gadgets/biguint.rs b/src/gadgets/biguint.rs
index f064a1bd..3aa5c8c1 100644
--- a/src/gadgets/biguint.rs
+++ b/src/gadgets/biguint.rs
@@ -104,7 +104,7 @@ impl<F: RichField + Extendable<D>, const D: usize> CircuitBuilder<F, D> {
             carry = new_carry;
             combined_limbs.push(new_limb);
         }
-        combined_limbs[num_limbs] = carry;
+        combined_limbs.push(carry);
 
         BigUintTarget {
             limbs: combined_limbs,