diff --git a/docs/api.md b/docs/api.md
index b41d8b52f9..f8ea73cfb7 100644
--- a/docs/api.md
+++ b/docs/api.md
@@ -3365,7 +3365,7 @@ Whether this request is driving frame's navigation.
 - returns: <[string]> Request's method (GET, POST, etc.)
 
 #### request.postData()
-- returns: <[string]> Request's post body, if any.
+- returns: <?[string]> Request's post body, if any.
 
 #### request.redirectChain()
 - returns: <[Array]<[Request]>>
diff --git a/src/chromium/crNetworkManager.ts b/src/chromium/crNetworkManager.ts
index 514d5e769e..2bccb6a14d 100644
--- a/src/chromium/crNetworkManager.ts
+++ b/src/chromium/crNetworkManager.ts
@@ -255,7 +255,7 @@ class InterceptableRequest implements network.RequestDelegate {
     this._documentId = documentId;
 
     this.request = new network.Request(allowInterception ? this : null, frame, redirectChain, documentId,
-        event.request.url, (event.type || '').toLowerCase(), event.request.method, event.request.postData, headersObject(event.request.headers));
+        event.request.url, (event.type || '').toLowerCase(), event.request.method, event.request.postData || null, headersObject(event.request.headers));
   }
 
   async continue(overrides: { method?: string; headers?: network.Headers; postData?: string } = {}) {
diff --git a/src/firefox/ffNetworkManager.ts b/src/firefox/ffNetworkManager.ts
index b3608c5612..654d97c47b 100644
--- a/src/firefox/ffNetworkManager.ts
+++ b/src/firefox/ffNetworkManager.ts
@@ -155,7 +155,7 @@ class InterceptableRequest implements network.RequestDelegate {
       headers[name.toLowerCase()] = value;
 
     this.request = new network.Request(payload.isIntercepted ? this : null, frame, redirectChain, payload.navigationId,
-        payload.url, causeToResourceType[payload.cause] || 'other', payload.method, payload.postData, headers);
+        payload.url, causeToResourceType[payload.cause] || 'other', payload.method, payload.postData || null, headers);
   }
 
   async continue(overrides: { method?: string; headers?: network.Headers; postData?: string }) {
diff --git a/src/network.ts b/src/network.ts
index 2ddd8b64f6..5aa30047fa 100644
--- a/src/network.ts
+++ b/src/network.ts
@@ -105,7 +105,7 @@ export class Request {
   private _url: string;
   private _resourceType: string;
   private _method: string;
-  private _postData: string | undefined;
+  private _postData: string | null;
   private _headers: Headers;
   private _frame: frames.Frame;
   private _waitForResponsePromise: Promise<Response>;
@@ -115,7 +115,7 @@ export class Request {
   private _interceptionHandled = false;
 
   constructor(delegate: RequestDelegate | null, frame: frames.Frame, redirectChain: Request[], documentId: string | undefined,
-    url: string, resourceType: string, method: string, postData: string | undefined, headers: Headers) {
+    url: string, resourceType: string, method: string, postData: string | null, headers: Headers) {
     assert(!url.startsWith('data:'), 'Data urls should not fire requests');
     this._delegate = delegate;
     this._frame = frame;
@@ -151,7 +151,7 @@ export class Request {
     return this._method;
   }
 
-  postData(): string | undefined {
+  postData(): string | null {
     return this._postData;
   }
 
diff --git a/src/webkit/wkInterceptableRequest.ts b/src/webkit/wkInterceptableRequest.ts
index 96be3b5ad2..afa46ba197 100644
--- a/src/webkit/wkInterceptableRequest.ts
+++ b/src/webkit/wkInterceptableRequest.ts
@@ -50,7 +50,7 @@ export class WKInterceptableRequest implements network.RequestDelegate {
     this._session = session;
     this._requestId = event.requestId;
     this.request = new network.Request(allowInterception ? this : null, frame, redirectChain, documentId, event.request.url,
-        event.type ? event.type.toLowerCase() : 'Unknown', event.request.method, event.request.postData, headersObject(event.request.headers));
+        event.type ? event.type.toLowerCase() : 'Unknown', event.request.method, event.request.postData || null, headersObject(event.request.headers));
     this._interceptedPromise = new Promise(f => this._interceptedCallback = f);
   }
 
diff --git a/test/browsercontext.spec.js b/test/browsercontext.spec.js
index 2c53a45408..3b660dd478 100644
--- a/test/browsercontext.spec.js
+++ b/test/browsercontext.spec.js
@@ -376,7 +376,7 @@ module.exports.describe = function({testRunner, expect, playwright, CHROMIUM, FF
         expect(request.url()).toContain('empty.html');
         expect(request.headers()['user-agent']).toBeTruthy();
         expect(request.method()).toBe('GET');
-        expect(request.postData()).toBe(undefined);
+        expect(request.postData()).toBe(null);
         expect(request.isNavigationRequest()).toBe(true);
         expect(request.resourceType()).toBe('document');
         expect(request.frame() === page.mainFrame()).toBe(true);
diff --git a/test/interception.spec.js b/test/interception.spec.js
index 2680d36563..93a81f32d1 100644
--- a/test/interception.spec.js
+++ b/test/interception.spec.js
@@ -36,7 +36,7 @@ module.exports.describe = function({testRunner, expect, defaultBrowserOptions, p
         expect(request.url()).toContain('empty.html');
         expect(request.headers()['user-agent']).toBeTruthy();
         expect(request.method()).toBe('GET');
-        expect(request.postData()).toBe(undefined);
+        expect(request.postData()).toBe(null);
         expect(request.isNavigationRequest()).toBe(true);
         expect(request.resourceType()).toBe('document');
         expect(request.frame() === page.mainFrame()).toBe(true);
@@ -586,7 +586,7 @@ module.exports.describe = function({testRunner, expect, defaultBrowserOptions, p
         expect(request.url()).toContain('empty.html');
         expect(request.headers()['user-agent']).toBeTruthy();
         expect(request.method()).toBe('GET');
-        expect(request.postData()).toBe(undefined);
+        expect(request.postData()).toBe(null);
         expect(request.isNavigationRequest()).toBe(true);
         expect(request.resourceType()).toBe('document');
         expect(request.frame() === page.mainFrame()).toBe(true);
diff --git a/test/network.spec.js b/test/network.spec.js
index 883fd82a87..7fa4ece158 100644
--- a/test/network.spec.js
+++ b/test/network.spec.js
@@ -112,7 +112,7 @@ module.exports.describe = function({testRunner, expect, MAC, WIN, FFOX, CHROMIUM
     });
     it('should be |undefined| when there is no post data', async({page, server}) => {
       const response = await page.goto(server.EMPTY_PAGE);
-      expect(response.request().postData()).toBe(undefined);
+      expect(response.request().postData()).toBe(null);
     });
   });