diff --git a/jest.config.ts b/jest.config.ts
index 4c713a0..e4bdc06 100644
--- a/jest.config.ts
+++ b/jest.config.ts
@@ -3,6 +3,7 @@ import type { JestConfigWithTsJest } from 'ts-jest'
 const jestConfig: JestConfigWithTsJest = {
   testEnvironment: 'node',
   testMatch: ['**/?(*.)+(spec|test).ts?(x)'],
+  setupFilesAfterEnv: ['<rootDir>/src/__tests__/setup.ts'],
   transform: {
     '^.+\\.[tj]s?$': [
       'ts-jest',
diff --git a/src/__tests__/setup.ts b/src/__tests__/setup.ts
new file mode 100644
index 0000000..7410b3f
--- /dev/null
+++ b/src/__tests__/setup.ts
@@ -0,0 +1,18 @@
+import MockAdapter from 'axios-mock-adapter';
+import type { AxiosRequestConfig } from 'axios';
+
+/**
+ * axios >= 0.33 hardened its internal config/header merge to build null-prototype objects.
+ * axios-mock-adapter reads `config.headers.constructor.name` to detect AxiosHeaders, which
+ * throws on such objects. Hand the mock adapter a normal-prototype copy of the headers.
+ *
+ * @see https://github.com/ctimmerm/axios-mock-adapter/blob/master/src/handle_request.js
+ */
+const originalAdapter = MockAdapter.prototype.adapter;
+MockAdapter.prototype.adapter = function patchedAdapter(this: MockAdapter) {
+  const handleRequest = originalAdapter.call(this) as (config: AxiosRequestConfig) => Promise<unknown>;
+  return ((config: AxiosRequestConfig) =>
+    config.headers && Object.getPrototypeOf(config.headers) === null
+      ? handleRequest({ ...config, headers: { ...config.headers } })
+      : handleRequest(config)) as ReturnType<typeof originalAdapter>;
+};
diff --git a/src/client.test.ts b/src/client.test.ts
index ef6e32f..1de5be7 100644
--- a/src/client.test.ts
+++ b/src/client.test.ts
@@ -281,7 +281,7 @@ describe('OpenAPIClientAxios', () => {
       expect(d.timeout).toBe(1234);
       expect(d.withCredentials).toBe(true);
       expect(d.adapter).toBe(userAdapter);
-      expect(d.auth).toStrictEqual({
+      expect(d.auth).toEqual({
         username: 'fake',
         password: 'fakepassword'
       }),
@@ -296,7 +296,7 @@ describe('OpenAPIClientAxios', () => {
       expect(d.validateStatus).toBe(userValidateStatus);
       expect(d.maxRedirects).toBe(99);
       expect(d.socketPath).toBe('/fake/path/example');
-      expect(d.proxy).toStrictEqual({
+      expect(d.proxy).toEqual({
           host: '1.2.3.4',
           port: 9876,
           auth: {