--- a/js/src/shell/js.cpp
+++ b/js/src/shell/js.cpp
@@ -6220,36 +6220,48 @@ EnsureGrayRoot(JSContext* cx, unsigned a
     return true;
 }
 
 static MarkBitObservers*
 EnsureMarkBitObservers(JSContext* cx)
 {
     ShellContext* sc = GetShellContext(cx);
     if (!sc->markObservers) {
-        sc->markObservers.reset(cx->new_<MarkBitObservers>(cx->runtime(),
-                                                         NonshrinkingGCObjectVector()));
+        auto* observers =
+            cx->new_<MarkBitObservers>(cx->runtime(), NonshrinkingGCObjectVector());
+        if (!observers)
+            return nullptr;
+        sc->markObservers.reset(observers);
     }
     return sc->markObservers.get();
 }
 
 static bool
 ClearMarkObservers(JSContext* cx, unsigned argc, Value* vp)
 {
+    CallArgs args = CallArgsFromVp(argc, vp);
+
     auto markObservers = EnsureMarkBitObservers(cx);
+    if (!markObservers)
+        return false;
+
     markObservers->get().clear();
+
+    args.rval().setUndefined();
     return true;
 }
 
 static bool
 AddMarkObservers(JSContext* cx, unsigned argc, Value* vp)
 {
     CallArgs args = CallArgsFromVp(argc, vp);
 
     auto markObservers = EnsureMarkBitObservers(cx);
+    if (!markObservers)
+        return false;
 
     if (!args.get(0).isObject()) {
         JS_ReportErrorASCII(cx, "argument must be an Array of objects");
         return false;
     }
 
     // WeakCaches are not swept during a minor GC. To prevent nursery-allocated
     // contents from having the mark bits be deceptively black until the second
@@ -7094,16 +7106,44 @@ JS_FN_HELP("parseBin", BinParse, 1, 0,
 "Take jobs from the shell's job queue in FIFO order and run them until the\n"
 "queue is empty.\n"),
 
     JS_FN_HELP("setPromiseRejectionTrackerCallback", SetPromiseRejectionTrackerCallback, 1, 0,
 "setPromiseRejectionTrackerCallback()",
 "Sets the callback to be invoked whenever a Promise rejection is unhandled\n"
 "or a previously-unhandled rejection becomes handled."),
 
+    JS_FN_HELP("dumpScopeChain", DumpScopeChain, 1, 0,
+"dumpScopeChain(obj)",
+"  Prints the scope chain of an interpreted function or a module."),
+
+    JS_FN_HELP("grayRoot", EnsureGrayRoot, 0, 0,
+"grayRoot()",
+"  Create a gray root Array, if needed, for the current compartment, and\n"
+"  return it."),
+
+    JS_FN_HELP("addMarkObservers", AddMarkObservers, 1, 0,
+"addMarkObservers(array_of_objects)",
+"  Register an array of objects whose mark bits will be tested by calls to\n"
+"  getMarks. The objects will be in calling compartment. Objects from\n"
+"  multiple compartments may be monitored by calling this function in\n"
+"  different compartments."),
+
+    JS_FN_HELP("clearMarkObservers", ClearMarkObservers, 1, 0,
+"clearMarkObservers()",
+"  Clear out the list of objects whose mark bits will be tested.\n"),
+
+    JS_FN_HELP("getMarks", GetMarks, 0, 0,
+"getMarks()",
+"  Return an array of strings representing the current state of the mark\n"
+"  bits ('gray' or 'black', or 'dead' if the object has been collected)\n"
+"  for the objects registered via addMarkObservers. Note that some of the\n"
+"  objects tested may be from different compartments than the one in which\n"
+"  this function runs."),
+
     JS_FN_HELP("bindToAsyncStack", BindToAsyncStack, 2, 0,
 "bindToAsyncStack(fn, { stack, cause, explicit })",
 "  Returns a new function that calls 'fn' with no arguments, passing\n"
 "  'undefined' as the 'this' value, and supplies an async stack for the\n"
 "  call as described by the second argument, an object with the following\n"
 "  properties (which are not optional, unless specified otherwise):\n"
 "\n"
 "  stack:    A SavedFrame object, like that returned by 'saveStack'. Stacks\n"
@@ -7185,44 +7225,16 @@ TestAssertRecoveredOnBailout,
 "  performed, and which, presumably, only |hook| knows how to find.\n"),
 
     JS_FN_HELP("trackedOpts", ReflectTrackedOptimizations, 1, 0,
 "trackedOpts(fun)",
 "  Returns an object describing the tracked optimizations of |fun|, if\n"
 "  any. If |fun| is not a scripted function or has not been compiled by\n"
 "  Ion, null is returned."),
 
-    JS_FN_HELP("dumpScopeChain", DumpScopeChain, 1, 0,
-"dumpScopeChain(obj)",
-"  Prints the scope chain of an interpreted function or a module."),
-
-    JS_FN_HELP("grayRoot", EnsureGrayRoot, 0, 0,
-"grayRoot()",
-"  Create a gray root Array, if needed, for the current compartment, and\n"
-"  return it."),
-
-    JS_FN_HELP("addMarkObservers", AddMarkObservers, 1, 0,
-"addMarkObservers(array_of_objects)",
-"  Register an array of objects whose mark bits will be tested by calls to\n"
-"  getMarks. The objects will be in calling compartment. Objects from\n"
-"  multiple compartments may be monitored by calling this function in\n"
-"  different compartments."),
-
-    JS_FN_HELP("clearMarkObservers", ClearMarkObservers, 1, 0,
-"clearMarkObservers()",
-"  Clear out the list of objects whose mark bits will be tested.\n"),
-
-    JS_FN_HELP("getMarks", GetMarks, 0, 0,
-"getMarks()",
-"  Return an array of strings representing the current state of the mark\n"
-"  bits ('gray' or 'black', or 'dead' if the object has been collected)\n"
-"  for the objects registered via addMarkObservers. Note that some of the\n"
-"  objects tested may be from different compartments than the one in which\n"
-"  this function runs."),
-
     JS_FN_HELP("crash", Crash, 0, 0,
 "crash([message, [{disable_minidump:true}]])",
 "  Crashes the process with a MOZ_CRASH, optionally providing a message.\n"
 "  An options object may be passed as the second argument. If the key\n"
 "  'suppress_minidump' is set to true, then a minidump will not be\n"
 "  generated by the crash (which only has an effect if the breakpad\n"
 "  dumping library is loaded.)"),