diff --git a/circuits/multiplier.circom b/circuits/multiplier.circom
index ee13cad..64b8413 100644
--- a/circuits/multiplier.circom
+++ b/circuits/multiplier.circom
@@ -1,14 +1,33 @@
 pragma circom 2.0.0;
 
+template IsZero() {
+  signal input in;
+  signal output out;
+
+  signal inv;
+  inv <-- in != 0 ? (1 / in) : 0;
+
+  out <== (-in * inv) + 1;
+  in * out === 0;
+}
+
 template Multiplier(n) {
   assert(n > 1);
   signal input in[n];
   signal output out;
 
-  signal inner[n-1];
+  // assert that all numbers are != 1
+  component isZero[n];
+  for (var i = 0; i < n; i++) {
+    isZero[i] = IsZero();
+    isZero[i].in <== in[i] - 1;
+    isZero[i].out === 0;
+  }  
 
+  // multiply
+  signal inner[n-1];
   inner[0] <== in[0] * in[1];
-  for(var i = 2; i < n; i++) {
+  for (var i = 2; i < n; i++) {
     inner[i-1] <== inner[i-2] * in[i];
   }
 
diff --git a/src/testers/witnessTester.ts b/src/testers/witnessTester.ts
index 7c15094..3b1d07b 100644
--- a/src/testers/witnessTester.ts
+++ b/src/testers/witnessTester.ts
@@ -15,9 +15,7 @@ export default class WitnessTester<IN extends readonly string[] = [], OUT extend
     this.circomWasmTester = circomWasmTester;
   }
 
-  /** Assert that constraints are valid for a given witness.
-   * @param witness witness
-   */
+  /** Assert that constraints are valid for a given witness. */
   async expectConstraintPass(witness: WitnessType): Promise<void> {
     return this.circomWasmTester.checkConstraints(witness);
   }
@@ -27,18 +25,18 @@ export default class WitnessTester<IN extends readonly string[] = [], OUT extend
    * This is useful to test if a fake witness (a witness from a
    * dishonest prover) can still be valid, which would indicate
    * that there are soundness errors in the circuit.
-   * @param witness witness
    */
   async expectConstraintFail(witness: WitnessType): Promise<void> {
     await this.expectConstraintPass(witness).then(
       () => assert.fail('Expected constraints to not match.'),
-      err => expect(err).to.be.instanceOf(AssertionError)
+      err => {
+        // console.log(err.message);
+        expect(err.message).to.eq("Constraint doesn't match");
+      }
     );
   }
 
-  /** Compute witness given the input signals.
-   * @param input all signals, private and public
-   */
+  /** Compute witness given the input signals. */
   async calculateWitness(input: CircuitSignals<IN>): Promise<WitnessType> {
     return this.circomWasmTester.calculateWitness(input, true);
   }
@@ -74,7 +72,10 @@ export default class WitnessTester<IN extends readonly string[] = [], OUT extend
   async expectFail(input: CircuitSignals<IN>) {
     await this.calculateWitness(input).then(
       () => assert.fail('Expected witness calculation to fail.'),
-      err => expect(err).to.be.instanceOf(AssertionError)
+      err => {
+        // console.log(err.message);
+        expect(err.message.startsWith('Error: Assert Failed.')).to.be.true;
+      }
     );
   }
 
diff --git a/tests/common/index.ts b/tests/common/index.ts
index df0ceb0..a863f7d 100644
--- a/tests/common/index.ts
+++ b/tests/common/index.ts
@@ -3,6 +3,8 @@ export const N = 3;
 
 const numbers = Array.from({length: N}, () => Math.floor(Math.random() * 100 * N));
 const product = numbers.reduce((prev, acc) => acc * prev);
+const malicious = Array.from({length: N}, () => 1);
+malicious[0] = product;
 
 export const CIRCUIT_NAME = `multiplier_${N}`;
 export const CIRCUIT_CONFIG = {
@@ -16,6 +18,9 @@ export const PTAU_PATH = './ptau/powersOfTau28_hez_final_08.ptau';
 export const INPUT = {
   in: numbers,
 };
+export const BAD_INPUT = {
+  in: malicious,
+};
 export const OUTPUT = {
   out: product,
 };
diff --git a/tests/testers.test.ts b/tests/testers.test.ts
index 9335483..d6ded8b 100644
--- a/tests/testers.test.ts
+++ b/tests/testers.test.ts
@@ -1,5 +1,5 @@
 import {Circomkit, ProofTester, WitnessTester} from '../src';
-import {CIRCUIT_CONFIG, CIRCUIT_NAME, INPUT, N, OUTPUT} from './common';
+import {BAD_INPUT, CIRCUIT_CONFIG, CIRCUIT_NAME, INPUT, N, OUTPUT} from './common';
 import {expect} from 'chai';
 
 describe('witness tester', () => {
@@ -14,13 +14,21 @@ describe('witness tester', () => {
   });
 
   it('should have correct number of constraints', async () => {
-    await circuit.expectConstraintCount(N, true);
+    // N - 1 constraints for each multiplication
+    // 1 constraint for the output
+    // 4 * N constraints to check if each input is different than 1
+    // TOTAL: 5 * N
+    await circuit.expectConstraintCount(5 * N, true);
   });
 
   it('should assert correctly', async () => {
     await circuit.expectPass(INPUT, OUTPUT);
   });
 
+  it('should fail for bad inupt', async () => {
+    await circuit.expectFail(BAD_INPUT);
+  });
+
   it('should compute correctly', async () => {
     const output = await circuit.compute(INPUT, ['out']);
     expect(output).to.haveOwnProperty('out');
@@ -35,7 +43,7 @@ describe('witness tester', () => {
   it('should NOT assert for bad witness', async () => {
     const witness = await circuit.calculateWitness(INPUT);
     const badWitness = await circuit.editWitness(witness, {
-      'main.inner[0]': 99999n,
+      'main.inner[0]': 1n,
     });
     await circuit.expectConstraintFail(badWitness);
   });