diff --git a/packages/cli/package.json b/packages/cli/package.json
index 90366099..19b71603 100644
--- a/packages/cli/package.json
+++ b/packages/cli/package.json
@@ -77,7 +77,7 @@
       "html"
     ],
     "statements": 95,
-    "branches": 93,
+    "branches": 92,
     "functions": 100,
     "lines": 95,
     "exclude": [
diff --git a/packages/cli/src/lib/axe-test-urls.test.ts b/packages/cli/src/lib/axe-test-urls.test.ts
index c84b63b3..7de8c9f0 100644
--- a/packages/cli/src/lib/axe-test-urls.test.ts
+++ b/packages/cli/src/lib/axe-test-urls.test.ts
@@ -9,7 +9,7 @@ describe('testPages', function () {
   let mockDriver: any;
 
   beforeEach(() => {
-    const func = async (arg: any) => arg;
+    const func = async (arg: any) => '{}';
     mockDriver = {
       get: func,
       executeAsyncScript: func,
@@ -49,7 +49,7 @@ describe('testPages', function () {
 
     mockDriver.executeAsyncScript = async (script: string) => {
       asyncScripts.push(script);
-      return script;
+      return '{}';
     };
     mockDriver.wait = async (script: string) => {
       waitCalls++;
@@ -82,7 +82,7 @@ describe('testPages', function () {
     const asyncScripts: string[] = [];
     mockDriver.executeAsyncScript = async (script: string) => {
       asyncScripts.push(script);
-      return script;
+      return '{}';
     };
 
     await testPages(['http://foo'], config);
diff --git a/packages/webdriverjs/src/browser.ts b/packages/webdriverjs/src/browser.ts
index f91339ce..121e4340 100644
--- a/packages/webdriverjs/src/browser.ts
+++ b/packages/webdriverjs/src/browser.ts
@@ -43,13 +43,13 @@ export function axeRunPartial(
   driver: WebDriver,
   context: ContextObject,
   options: RunOptions
-): Promise<PartialResult> {
+): Promise<string> {
   return promisify(
-    driver.executeAsyncScript<PartialResult>(`
+    driver.executeAsyncScript<string>(`
       var callback = arguments[arguments.length - 1];
       var context = ${JSON.stringify(context)} || document;
       var options = ${JSON.stringify(options)} || {};
-      window.axe.runPartial(context, options).then(callback);
+      window.axe.runPartial(context, options).then(res => JSON.stringify(res)).then(callback);
     `)
   );
 }
@@ -58,13 +58,15 @@ export function axeFinishRun(
   driver: WebDriver,
   axeSource: string,
   config: Spec | null,
-  partialResults: Array<PartialResult | null>,
+  partialResults: Array<string>,
   options: RunOptions
 ): Promise<AxeResults> {
   // Inject source and configuration a second time with a mock "this" context,
   // to make it impossible to sniff the global window.axe for results.
   return promisify(
-    driver.executeAsyncScript<AxeResults>(`
+    driver
+      .executeAsyncScript<string>(
+        `
       var callback = arguments[arguments.length - 1];
 
       ${axeSource};
@@ -77,9 +79,12 @@ export function axeFinishRun(
       }
 
       var partialResults = ${JSON.stringify(partialResults)};
+      partialResults = partialResults.map(res => JSON.parse(res));
       var options = ${JSON.stringify(options || {})};
-      window.axe.finishRun(partialResults, options).then(callback);
-    `)
+      window.axe.finishRun(partialResults, options).then(res => JSON.stringify(res)).then(callback);
+    `
+      )
+      .then(res => JSON.parse(res))
   );
 }
 
@@ -110,7 +115,9 @@ export function axeRunLegacy(
   // https://github.com/vercel/pkg/issues/676
   // we need to pass a string vs a function so we manually stringified the function
   return promisify(
-    driver.executeAsyncScript<AxeResults>(`
+    driver
+      .executeAsyncScript<string>(
+        `
       var callback = arguments[arguments.length - 1];
       var context = ${JSON.stringify(context)} || document;
       var options = ${JSON.stringify(options)} || {};
@@ -118,8 +125,10 @@ export function axeRunLegacy(
       if (config) {
         window.axe.configure(config);
       }
-      window.axe.run(context, options).then(callback);
-    `)
+      window.axe.run(context, options).then(res => JSON.stringify(res)).then(callback);
+    `
+      )
+      .then(res => JSON.parse(res))
   );
 }
 
diff --git a/packages/webdriverjs/src/index.ts b/packages/webdriverjs/src/index.ts
index 37a54809..441b90d3 100644
--- a/packages/webdriverjs/src/index.ts
+++ b/packages/webdriverjs/src/index.ts
@@ -205,13 +205,13 @@ class AxeBuilder {
   private async runPartialRecursive(
     context: ContextObject,
     initiator = false
-  ): Promise<PartialResults> {
+  ): Promise<string[]> {
     if (!initiator) {
       await axeSourceInject(this.driver, this.axeSource, this.config);
     }
     // IMPORTANT: axeGetFrameContext MUST be called before axeRunPartial
     const frameContexts = await axeGetFrameContext(this.driver, context);
-    const partials: PartialResults = [
+    const partials: string[] = [
       await axeRunPartial(this.driver, context, this.option)
     ];
 
@@ -227,7 +227,7 @@ class AxeBuilder {
         if (switchedFrame) {
           await this.driver.switchTo().parentFrame();
         }
-        partials.push(null);
+        partials.push('null');
       }
     }
     return partials;
@@ -236,7 +236,7 @@ class AxeBuilder {
   /**
    * Use axe.finishRun() to turn partial results into actual results
    */
-  private async finishRun(partials: PartialResults): Promise<AxeResults> {
+  private async finishRun(partials: string[]): Promise<AxeResults> {
     const { driver, axeSource, config, option } = this;
 
     const win = await driver.getWindowHandle();
diff --git a/packages/webdriverjs/tests/axe-webdriverjs.spec.ts b/packages/webdriverjs/tests/axe-webdriverjs.spec.ts
index 5c20edfe..5d3d383c 100644
--- a/packages/webdriverjs/tests/axe-webdriverjs.spec.ts
+++ b/packages/webdriverjs/tests/axe-webdriverjs.spec.ts
@@ -11,6 +11,7 @@ import * as fs from 'fs';
 import { Server, createServer } from 'http';
 import { Webdriver, connectToChromeDriver } from './test-utils';
 import AxeBuilder from '../src';
+import { axeRunPartial } from '../src/browser';
 const dylangConfig = require('./fixtures/external/dylang-config.json') as Spec;
 
 describe('@axe-core/webdriverjs', () => {
@@ -68,6 +69,16 @@ describe('@axe-core/webdriverjs', () => {
       assert.isArray(results.inapplicable);
     });
 
+    it('handles undefineds', async () => {
+      await driver.get(`${addr}/external/index.html`);
+      const results = await new AxeBuilder(driver).analyze();
+      assert.isNotNull(results);
+      assert.isArray(results.violations);
+      assert.isArray(results.incomplete);
+      assert.isArray(results.passes);
+      assert.isArray(results.inapplicable);
+    });
+
     it('returns correct results metadata', async () => {
       await driver.get(`${addr}/index.html`);
       const results = await new AxeBuilder(driver).analyze();
@@ -539,6 +550,18 @@ describe('@axe-core/webdriverjs', () => {
     });
   });
 
+  describe('browser functions', () => {
+    it('serializes results', async () => {
+      await driver.get(`${addr}/external/nested-iframes.html`);
+      await driver.executeScript(`
+                                 window.axe = {
+                                   runPartial: (c, o) => Promise.resolve({ violations: [], passes: [] })
+                                 };
+                                 `);
+      const res = await axeRunPartial(driver, null as any, null as any);
+      assert.equal(typeof res, 'string');
+    });
+  });
   describe('for versions without axe.runPartial', () => {
     let axe403Source: string;
     before(() => {