diff --git a/src/ivc/sangria/fold_relaxed_plonk_instance_chip.rs b/src/ivc/sangria/fold_relaxed_plonk_instance_chip.rs
index 19cab9d3..0614660a 100644
--- a/src/ivc/sangria/fold_relaxed_plonk_instance_chip.rs
+++ b/src/ivc/sangria/fold_relaxed_plonk_instance_chip.rs
@@ -898,7 +898,12 @@ where
 
         let assigned_step_circuit_instances =
             match self.relaxed.step_circuit_instances_hash_accumulator {
-                SCInstancesHashAcc::None => SCInstancesHashAcc::None,
+                SCInstancesHashAcc::None => {
+                    assign_and_absorb_diff_field!(&C::Base::ZERO, || {
+                        "empty step_circuit_instances"
+                    })?;
+                    SCInstancesHashAcc::None
+                }
                 SCInstancesHashAcc::Hash(hash) => {
                     SCInstancesHashAcc::Hash(assign_and_absorb_diff_field!(&hash, || {
                         "step_circuit_instances"