diff --git a/types/firefox-webext-browser/firefox-webext-browser-tests.ts b/types/firefox-webext-browser/firefox-webext-browser-tests.ts
index 01d9cf9f3d..38169d573a 100644
--- a/types/firefox-webext-browser/firefox-webext-browser-tests.ts
+++ b/types/firefox-webext-browser/firefox-webext-browser-tests.ts
@@ -8,3 +8,7 @@ browser.manifest; // $ExpectError
 browser._manifest; // $ExpectType typeof _manifest
 browser._manifest.WebExtensionLangpackManifest; // $ExpectError
 browser._manifest.NativeManifest; // $ExpectError
+
+// browser.runtime
+const port = browser.runtime.connect();
+port.postMessage(); // $ExpectError
diff --git a/types/firefox-webext-browser/index.d.ts b/types/firefox-webext-browser/index.d.ts
index dcbfb48648..cba61923de 100644
--- a/types/firefox-webext-browser/index.d.ts
+++ b/types/firefox-webext-browser/index.d.ts
@@ -2299,7 +2299,7 @@ declare namespace browser.runtime {
         disconnect: () => void;
         onDisconnect: events.Event;
         onMessage: events.Event;
-        postMessage: () => void;
+        postMessage(message: object): void;
         /** This property will **only** be present on ports passed to onConnect/onConnectExternal listeners. */
         sender?: MessageSender;
     }