diff --git a/ceno_zkvm/src/uint/arithmetic.rs b/ceno_zkvm/src/uint/arithmetic.rs
index fe6dc88ef..2cd84d36c 100644
--- a/ceno_zkvm/src/uint/arithmetic.rs
+++ b/ceno_zkvm/src/uint/arithmetic.rs
@@ -254,7 +254,7 @@ impl<const M: usize, E: ExtensionField> UInt<M, 8, E> {
         circuit_builder.lookup_and_byte(
             high_limb_no_msb.expr(),
             high_limb.clone(),
-            Expression::from(0xFF),
+            Expression::from(0b0111_1111),
         )?;
 
         let inv_128 = F::from(128).invert().unwrap();