--- a/testing/marionette/server.js
+++ b/testing/marionette/server.js
@@ -429,24 +429,43 @@ server.TCPConnection = class {
    * a Command, the corresponding is executed.
    *
    * @param {Array.<number, number, ?, ?>} data
    *     A four element array where the elements, in sequence, signifies
    *     message type, message ID, method name or error, and parameters
    *     or result.
    */
   onPacket (data) {
-    let msg = Message.fromMsg(data);
-    msg.origin = MessageOrigin.Client;
-    this.log_(msg);
+    // unable to determine how to respond
+    if (!Array.isArray(data)) {
+      let e = new TypeError(
+          "Unable to unmarshal packet data: " + JSON.stringify(data));
+      error.report(e);
+      return;
+    }
 
+    // return immediately with any error trying to unmarshal message
+    let msg;
+    try {
+      msg = Message.fromMsg(data);
+      msg.origin = MessageOrigin.Client;
+      this.log_(msg);
+    } catch (e) {
+      let resp = this.createResponse(data[1]);
+      resp.sendError(e);
+      return;
+    }
+
+    // look up previous command we received a response for
     if (msg instanceof Response) {
       let cmd = this.commands_.get(msg.id);
       this.commands_.delete(msg.id);
       cmd.onresponse(msg);
+
+    // execute new command
     } else if (msg instanceof Command) {
       this.lastID = msg.id;
       this.execute(msg);
     }
   }
 
   /**
    * Executes a WebDriver command and sends back a response when it has
@@ -464,21 +483,21 @@ server.TCPConnection = class {
    * Errors thrown in commands are marshaled and sent back, and if they
    * are not WebDriverError instances, they are additionally propagated
    * and reported to {@code Components.utils.reportError}.
    *
    * @param {Command} cmd
    *     The requested command to execute.
    */
   execute (cmd) {
-    let resp = new Response(cmd.id, this.send.bind(this));
+    let resp = this.createResponse(cmd.id);
     let sendResponse = () => resp.sendConditionally(resp => !resp.sent);
     let sendError = resp.sendError.bind(resp);
 
-    let req = Task.spawn(function*() {
+    let req = Task.spawn(function* () {
       let fn = this.driver.commands[cmd.name];
       if (typeof fn == "undefined") {
         throw new UnknownCommandError(cmd.name);
       }
 
       if (cmd.name !== "newSession") {
         assert.session(this.driver);
       }
@@ -492,16 +511,32 @@ server.TCPConnection = class {
           resp.body = rv;
         }
       }
     }.bind(this));
 
     req.then(sendResponse, sendError).catch(error.report);
   }
 
+  /**
+   * Fail-safe creation of a new instance of |message.Response|.
+   *
+   * @param {?} msgID
+   *     Message ID to respond to.  If it is not a number, -1 is used.
+   *
+   * @return {message.Response}
+   *     Response to the message with |msgID|.
+   */
+  createResponse (msgID) {
+    if (typeof msgID != "number") {
+      msgID = -1;
+    }
+    return new Response(msgID, this.send.bind(this));
+  }
+
   sendError (err, cmdID) {
     let resp = new Response(cmdID, this.send.bind(this));
     resp.sendError(err);
   }
 
   /**
    * When a client connects we send across a JSON Object defining the
    * protocol level.