diff --git a/ceno_zkvm/src/circuit_builder.rs b/ceno_zkvm/src/circuit_builder.rs
index b5a8cd13b..80a437f26 100644
--- a/ceno_zkvm/src/circuit_builder.rs
+++ b/ceno_zkvm/src/circuit_builder.rs
@@ -420,8 +420,8 @@ impl<E: ExtensionField> ConstraintSystem<E> {
     pub fn namespace<NR: Into<String>, N: FnOnce() -> NR, T>(
         &mut self,
         name_fn: N,
-        cb: impl FnOnce(&mut ConstraintSystem<E>) -> Result<T, ZKVMError>,
-    ) -> Result<T, ZKVMError> {
+        cb: impl FnOnce(&mut ConstraintSystem<E>) -> T,
+    ) -> T {
         self.ns.push_namespace(name_fn().into());
         let t = cb(self);
         self.ns.pop_namespace();
diff --git a/ceno_zkvm/src/instructions/riscv/test.rs b/ceno_zkvm/src/instructions/riscv/test.rs
index 09456f269..6513cd1f7 100644
--- a/ceno_zkvm/src/instructions/riscv/test.rs
+++ b/ceno_zkvm/src/instructions/riscv/test.rs
@@ -16,19 +16,11 @@ fn test_multiple_opcode() {
     let mut cs = ConstraintSystem::new(|| "riscv");
     let _add_config = cs.namespace(
         || "add",
-        |cs| {
-            let mut circuit_builder = CircuitBuilder::<E>::new(cs);
-            let config = AddInstruction::construct_circuit(&mut circuit_builder);
-            Ok(config)
-        },
+        |cs| AddInstruction::construct_circuit(&mut CircuitBuilder::<E>::new(cs)),
     );
     let _sub_config = cs.namespace(
         || "sub",
-        |cs| {
-            let mut circuit_builder = CircuitBuilder::<E>::new(cs);
-            let config = SubInstruction::construct_circuit(&mut circuit_builder);
-            Ok(config)
-        },
+        |cs| SubInstruction::construct_circuit(&mut CircuitBuilder::<E>::new(cs)),
     );
     let param = Pcs::setup(1 << 10).unwrap();
     let (pp, _) = Pcs::trim(&param, 1 << 10).unwrap();