diff --git a/remote/Error.jsm b/remote/Error.jsm
index 3e888d00bfb5..75bb353f970a 100644
--- a/remote/Error.jsm
+++ b/remote/Error.jsm
@@ -100,7 +100,15 @@ class FatalError extends RemoteAgentError {
 class UnsupportedError extends RemoteAgentError {}
 
 /** The requested remote method does not exist. */
-class UnknownMethodError extends RemoteAgentError {}
+class UnknownMethodError extends RemoteAgentError {
+  constructor(domain, command = null) {
+    if (command) {
+      super(`${domain}.${command}`);
+    } else {
+      super(domain);
+    }
+  }
+}
 
 function formatError(error, {stack = false} = {}) {
   const els = [];
diff --git a/remote/sessions/ContentProcessSession.jsm b/remote/sessions/ContentProcessSession.jsm
index 30bd397862c4..a9930d8f0144 100644
--- a/remote/sessions/ContentProcessSession.jsm
+++ b/remote/sessions/ContentProcessSession.jsm
@@ -8,6 +8,7 @@ var EXPORTED_SYMBOLS = ["ContentProcessSession"];
 
 const {ContentProcessDomains} = ChromeUtils.import("chrome://remote/content/domains/ContentProcessDomains.jsm");
 const {Domains} = ChromeUtils.import("chrome://remote/content/domains/Domains.jsm");
+const {UnknownMethodError} = ChromeUtils.import("chrome://remote/content/Error.jsm");
 
 class ContentProcessSession {
   constructor(messageManager, browsingContext, content, docShell) {
@@ -61,14 +62,11 @@ class ContentProcessSession {
     case "remote:request":
       try {
         const {id, domain, command, params} = data.request;
-
-        const inst = this.domains.get(domain);
-        const func = inst[command];
-        if (!func || typeof func != "function") {
-          throw new Error(`Implementation missing: ${domain}.${command}`);
+        if (!this.domains.domainSupportsMethod(domain, command)) {
+          throw new UnknownMethodError(domain, command);
         }
-
-        const result = await func.call(inst, params);
+        const inst = this.domains.get(domain);
+        const result = await inst[command](params);
 
         this.messageManager.sendAsyncMessage("remote:result", {
           browsingContextId,
diff --git a/remote/sessions/Session.jsm b/remote/sessions/Session.jsm
index 2dc6ff9fa6c5..46ffbd2f94c3 100644
--- a/remote/sessions/Session.jsm
+++ b/remote/sessions/Session.jsm
@@ -8,7 +8,10 @@ var EXPORTED_SYMBOLS = ["Session"];
 
 const {ParentProcessDomains} = ChromeUtils.import("chrome://remote/content/domains/ParentProcessDomains.jsm");
 const {Domains} = ChromeUtils.import("chrome://remote/content/domains/Domains.jsm");
-const {RemoteAgentError} = ChromeUtils.import("chrome://remote/content/Error.jsm");
+const {
+  RemoteAgentError,
+  UnknownMethodError,
+} = ChromeUtils.import("chrome://remote/content/Error.jsm");
 
 /**
  * A session represents exactly one client WebSocket connection.
@@ -69,6 +72,9 @@ class Session {
   }
 
   async execute(id, domain, command, params) {
+    if (!this.domains.domainSupportsMethod(domain, command)) {
+      throw new UnknownMethodError(domain, command);
+    }
     const inst = this.domains.get(domain);
     const result = await inst[command](params);
     this.onResult(id, result);
diff --git a/remote/test/browser/browser.ini b/remote/test/browser/browser.ini
index a8df818e5bc0..e16474e93c5a 100644
--- a/remote/test/browser/browser.ini
+++ b/remote/test/browser/browser.ini
@@ -16,5 +16,6 @@ support-files =
 [browser_runtime_callFunctionOn.js]
 [browser_runtime_executionContext.js]
 skip-if = os == "mac" || (verify && os == 'win') # bug 1547961
+[browser_session.js]
 [browser_tabs.js]
 [browser_target.js]
diff --git a/remote/test/browser/browser_session.js b/remote/test/browser/browser_session.js
new file mode 100644
index 000000000000..18c63bf0b909
--- /dev/null
+++ b/remote/test/browser/browser_session.js
@@ -0,0 +1,25 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+add_task(async function() {
+  await RemoteAgent.listen(Services.io.newURI("http://localhost:9222"));
+  const CDP = await getCDP();
+  const {webSocketDebuggerUrl} = await CDP.Version();
+  const client = await CDP({"target": webSocketDebuggerUrl});
+
+  try {
+    await client.send("Hoobaflooba");
+  } catch (e) {
+    ok(e.message.match(/TypeError: Invalid method format/));
+  }
+  try {
+    await client.send("Hooba.flooba");
+  } catch (e) {
+    ok(e.message.match(/UnknownMethodError/));
+  }
+
+  await client.close();
+  await RemoteAgent.close();
+});
diff --git a/remote/test/unit/test_Error.js b/remote/test/unit/test_Error.js
index 1b080f7a3201..ec45fdca1273 100644
--- a/remote/test/unit/test_Error.js
+++ b/remote/test/unit/test_Error.js
@@ -100,5 +100,7 @@ add_test(function test_UnsupportedError() {
 
 add_test(function test_UnknownMethodError() {
   ok(new UnknownMethodError() instanceof RemoteAgentError);
+  ok(new UnknownMethodError("domain").message.endsWith("domain"));
+  ok(new UnknownMethodError("domain", "command").message.endsWith("domain.command"));
   run_next_test();
 });