diff --git a/halo2_gadgets/src/utilities/decompose_running_sum.rs b/halo2_gadgets/src/utilities/decompose_running_sum.rs
index 5a83e417..1e86da03 100644
--- a/halo2_gadgets/src/utilities/decompose_running_sum.rs
+++ b/halo2_gadgets/src/utilities/decompose_running_sum.rs
@@ -194,11 +194,10 @@ impl<F: PrimeFieldBits, const WINDOW_NUM_BITS: usize> RunningSumConfig<F, WINDOW
             z = z_next;
             zs.push(z.clone());
         }
-        assert_eq!(zs.len(), num_windows + 1);
 
         if strict {
             // Constrain the final running sum output to be zero.
-            region.constrain_constant(zs.last().unwrap().cell(), F::ZERO)?;
+            region.constrain_constant(zs[num_windows].cell(), F::ZERO)?;
         }
 
         Ok(RunningSum(zs))