diff --git a/src/configs/index.ts b/src/configs/index.ts
index f5804f0..81bc76d 100644
--- a/src/configs/index.ts
+++ b/src/configs/index.ts
@@ -31,13 +31,14 @@ export type CircomkitConfig = {
   version: `${number}.${number}.${number}`;
   /**
    * [Optimization level](https://docs.circom.io/getting-started/compilation-options/#flags-and-options-related-to-the-r1cs-optimization).
-   * Defaults to `2` as per the Circom defaults, see [`circom/src/input_user.rs`](https://github.com/iden3/circom/blob/master/circom/src/input_user.rs#L249).
+   * See [`circom/src/input_user.rs`](https://github.com/iden3/circom/blob/master/circom/src/input_user.rs#L249).
+   * - `undefined`: Follow Circom default. (<v2.2.0: 2, >=v2.2.0: 1)
    * - `0`: No simplification is applied.
    * - `1`: Only applies `var` to `var` and `var` to `constant` simplification.
-   * - `2`: Full constraint simplificiation via Gaussian eliminations. (Default)
+   * - `2`: Full constraint simplificiation via Gaussian eliminations.
    * - `>2`: Any number higher than 2 will use `--O2round` with the number as simplification rounds.
    */
-  optimization: number;
+  optimization: number | undefined;
   /** Does an additional check over the constraints produced. */
   inspect: boolean;
   /** Include paths as libraries during compilation. */
@@ -66,7 +67,7 @@ export const DEFAULT = Object.seal<Readonly<CircomkitConfig>>({
   dirBuild: './build',
   circomPath: 'circom',
   // compiler-specific
-  optimization: 2,
+  optimization: undefined,
   inspect: true,
   include: ['./node_modules'],
   cWitness: false,
diff --git a/src/core/index.ts b/src/core/index.ts
index 8fa55db..81e74bd 100644
--- a/src/core/index.ts
+++ b/src/core/index.ts
@@ -65,7 +65,10 @@ export class Circomkit {
     if (!PROTOCOLS.includes(this.config.protocol)) {
       throw new Error('Invalid protocol in configuration.');
     }
-    if (this.config.optimization < 0) {
+    if (this.config.optimization !== undefined && typeof this.config.optimization !== 'number') {
+      throw new Error('Invalid optimization level.');
+    }
+    if (typeof this.config.optimization === 'number' && this.config.optimization < 0) {
       this.log.warn('Optimization level must be at least 0, setting it to 0.');
       this.config.optimization = 0;
     }
@@ -499,7 +502,7 @@ export class Circomkit {
       output: undefined, // this makes tests to be created under /tmp
       prime: this.config.prime,
       verbose: this.config.verbose,
-      O: Math.min(this.config.optimization, 1), // tester doesnt have O2
+      O: typeof this.config.optimization === 'number' ? Math.min(this.config.optimization, 1) : undefined, // tester doesnt have O2
       json: false,
       include: this.config.include,
       wasm: true,
diff --git a/src/functions/circuit.ts b/src/functions/circuit.ts
index 24322d0..3f349af 100644
--- a/src/functions/circuit.ts
+++ b/src/functions/circuit.ts
@@ -21,12 +21,14 @@ export async function compileCircuit(config: CircomkitConfig, targetPath: string
   if (config.verbose) flags += ' --verbose';
   if (config.inspect) flags += ' --inspect';
   if (config.cWitness) flags += ' --c';
-  if (config.optimization > 2) {
-    // --O2round <value>
-    flags += ` --O2round ${config.optimization}`;
-  } else {
-    // --O0, --O1 or --O2
-    flags += ` --O${config.optimization}`;
+  if (typeof config.optimization === 'number') {
+    if (config.optimization > 2) {
+      // --O2round <value>
+      flags += ` --O2round ${config.optimization}`;
+    } else {
+      // --O0, --O1 or --O2
+      flags += ` --O${config.optimization}`;
+    }
   }
 
   // call `circom` as a sub-process
diff --git a/tests/witnessTester.test.ts b/tests/witnessTester.test.ts
index 31676bc..02b2442 100644
--- a/tests/witnessTester.test.ts
+++ b/tests/witnessTester.test.ts
@@ -19,6 +19,7 @@ describe('witness tester', () => {
       dirCircuits: './tests/circuits',
       dirInputs: './tests/inputs',
       dirBuild: './tests/build',
+      optimization: 2,
     });
     circuit = await circomkit.WitnessTester(name, {...config, recompile: true});
   });