--- a/browser/devtools/debugger/test/browser.ini
+++ b/browser/devtools/debugger/test/browser.ini
@@ -262,16 +262,17 @@ skip-if = os == "linux" || e10s # Bug 88
 [browser_dbg_terminate-on-tab-close.js]
 [browser_dbg_tracing-01.js]
 [browser_dbg_tracing-02.js]
 [browser_dbg_tracing-03.js]
 [browser_dbg_tracing-04.js]
 [browser_dbg_tracing-05.js]
 [browser_dbg_tracing-06.js]
 [browser_dbg_tracing-07.js]
+[browser_dbg_tracing-08.js]
 [browser_dbg_variables-view-01.js]
 [browser_dbg_variables-view-02.js]
 [browser_dbg_variables-view-03.js]
 [browser_dbg_variables-view-04.js]
 [browser_dbg_variables-view-05.js]
 [browser_dbg_variables-view-accessibility.js]
 [browser_dbg_variables-view-data.js]
 [browser_dbg_variables-view-edit-cancel.js]

new file mode 100644
--- /dev/null
+++ b/browser/devtools/debugger/test/browser_dbg_tracing-08.js
@@ -0,0 +1,59 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/**
+ * Test that tracing about:config doesn't produce errors.
+ */
+
+const TAB_URL = "about:config";
+
+let gPanel, gDoneChecks;
+
+function test() {
+  gDoneChecks = promise.defer();
+  const tracerPref = promise.defer();
+  const configPref = promise.defer();
+  SpecialPowers.pushPrefEnv({'set': [["devtools.debugger.tracer", true]]}, tracerPref.resolve);
+  SpecialPowers.pushPrefEnv({'set': [["general.warnOnAboutConfig", false]]}, configPref.resolve);
+  promise.all([tracerPref.promise, configPref.promise]).then(() => {
+    initDebugger(TAB_URL).then(([,, aPanel]) => {
+      gPanel = aPanel;
+      gPanel.panelWin.gClient.addOneTimeListener("traces", testTraceLogs);
+    }).then(() => startTracing(gPanel))
+      .then(generateTrace)
+      .then(() => waitForClientEvents(gPanel, "traces"))
+      .then(() => gDoneChecks.promise)
+      .then(() => stopTracing(gPanel))
+      .then(resetPreferences)
+      .then(() => closeDebuggerAndFinish(gPanel))
+      .then(null, aError => {
+        ok(false, "Got an error: " + aError.message + "\n" + aError.stack);
+      });
+  });
+}
+
+function testTraceLogs(name, packet) {
+  info("Traces: " + packet.traces.length);
+  ok(packet.traces.length > 0, "Got some traces.");
+  ok(packet.traces.every(t => t.type != "enteredFrame" || !!t.location),
+     "All enteredFrame traces contain location.");
+  gDoneChecks.resolve();
+}
+
+function generateTrace(name, packet) {
+  // Interact with the page to cause JS execution.
+  let search = content.document.getElementById("textbox");
+  info("Interacting with the page.");
+  search.value = "devtools";
+}
+
+function resetPreferences() {
+  const deferred = promise.defer();
+  SpecialPowers.popPrefEnv(() => SpecialPowers.popPrefEnv(deferred.resolve));
+  return deferred.promise;
+}
+
+registerCleanupFunction(function() {
+  gPanel = null;
+  gDoneChecks = null;
+});

--- a/toolkit/devtools/server/actors/tracer.js
+++ b/toolkit/devtools/server/actors/tracer.js
@@ -101,17 +101,16 @@ TracerActor.prototype = {
 
   get attached() { return this._attached; },
   get idle()     { return this._attached && this._activeTraces.size === 0; },
   get tracing()  { return this._attached && this._activeTraces.size > 0; },
 
   get dbg() {
     if (!this._dbg) {
       this._dbg = this._parent.makeDebugger();
-      this._dbg.onEnterFrame = this.onEnterFrame;
     }
     return this._dbg;
   },
 
   /**
    * Buffer traces and only send them every BUFFER_SEND_DELAY milliseconds.
    */
   _send: function(aPacket) {
@@ -183,16 +182,17 @@ TracerActor.prototype = {
         return {
           error: "badParameterType",
           message: "No such trace type: " + traceType
         };
       }
     }
 
     if (this.idle) {
+      this.dbg.onEnterFrame = this.onEnterFrame;
       this.dbg.enabled = true;
       this._sequence = 0;
       this._startTime = Date.now();
     }
 
     // Start recording all requested trace types.
     for (let traceType of aRequest.trace) {
       this._requestsForTraceType[traceType]++;
@@ -239,16 +239,17 @@ TracerActor.prototype = {
     }
 
     // Clear hit counts if no trace is requesting them.
     if (!this._requestsForTraceType.hitCount) {
       this._hitCounts.clear();
     }
 
     if (this.idle) {
+      this._dbg.onEnterFrame = undefined;
       this.dbg.enabled = false;
     }
 
     return {
       type: "stoppedTrace",
       why: "requested",
       name
     };
@@ -259,21 +260,21 @@ TracerActor.prototype = {
   /**
    * Called by the engine when a frame is entered. Sends an unsolicited packet
    * to the client carrying requested trace information.
    *
    * @param aFrame Debugger.Frame
    *        The stack frame that was entered.
    */
   onEnterFrame: function(aFrame) {
+    if (aFrame.script && aFrame.script.url == "self-hosted") {
+      return;
+    }
+
     Task.spawn(function*() {
-      if (aFrame.script && aFrame.script.url == "self-hosted") {
-        return;
-      }
-
       // This function might request original (i.e. source-mapped) location,
       // which is asynchronous. We need to ensure that packets are sent out
       // in the correct order.
       let runInOrder = this._packetScheduler.schedule();
 
       let packet = {
         type: "enteredFrame",
         sequence: this._sequence++