diff --git a/runtime/vm/isolate.cc b/runtime/vm/isolate.cc
index 114918a..e3a60be 100644
--- a/runtime/vm/isolate.cc
+++ b/runtime/vm/isolate.cc
@@ -2854,7 +2854,19 @@
     scheduled_mutator_thread_->end_ = 0;
     scheduled_mutator_thread_ = NULL;
   }
-  thread->isolate_ = NULL;
+  // Even if we unschedule the mutator thread, e.g. via calling
+  // `Dart_ExitIsolate()` inside a native, we might still have one or more Dart
+  // stacks active, which e.g. GC marker threads want to visit.  So we don't
+  // clear out the isolate pointer if we are on the mutator thread.
+  //
+  // The [thread] structure for the mutator thread is kept alive in the thread
+  // registry even if the mutator thread is temporarily unscheduled.
+  //
+  // All other threads are not allowed to unschedule themselves and schedule
+  // again later on.
+  if (!is_mutator) {
+    thread->isolate_ = NULL;
+  }
   thread->heap_ = NULL;
   thread->set_os_thread(NULL);
   thread->set_execution_state(Thread::kThreadInNative);
diff --git a/runtime/vm/thread.cc b/runtime/vm/thread.cc
index 505b787..bc6c6ad 100644
--- a/runtime/vm/thread.cc
+++ b/runtime/vm/thread.cc
@@ -688,26 +688,32 @@
     scope = scope->previous();
   }
 
-  // The MarkTask, which calls this method, can run on a different thread.  We
-  // therefore assume the mutator is at a safepoint and we can iterate it's
-  // stack.
-  // TODO(vm-team): It would be beneficial to be able to ask the mutator thread
-  // whether it is in fact blocked at the moment (at a "safepoint") so we can
-  // safely iterate it's stack.
-  //
-  // Unfortunately we cannot use `this->IsAtSafepoint()` here because that will
-  // return `false` even though the mutator thread is waiting for mark tasks
-  // (which iterate it's stack) to finish.
-  const StackFrameIterator::CrossThreadPolicy cross_thread_policy =
-      StackFrameIterator::kAllowCrossThreadIteration;
+  // Only the mutator thread can run Dart code.
+  if (IsMutatorThread()) {
+    // The MarkTask, which calls this method, can run on a different thread.  We
+    // therefore assume the mutator is at a safepoint and we can iterate it's
+    // stack.
+    // TODO(vm-team): It would be beneficial to be able to ask the mutator
+    // thread whether it is in fact blocked at the moment (at a "safepoint") so
+    // we can safely iterate it's stack.
+    //
+    // Unfortunately we cannot use `this->IsAtSafepoint()` here because that
+    // will return `false` even though the mutator thread is waiting for mark
+    // tasks (which iterate it's stack) to finish.
+    const StackFrameIterator::CrossThreadPolicy cross_thread_policy =
+        StackFrameIterator::kAllowCrossThreadIteration;
 
-  // Iterate over all the stack frames and visit objects on the stack.
-  StackFrameIterator frames_iterator(top_exit_frame_info(), validation_policy,
-                                     this, cross_thread_policy);
-  StackFrame* frame = frames_iterator.NextFrame();
-  while (frame != NULL) {
-    frame->VisitObjectPointers(visitor);
-    frame = frames_iterator.NextFrame();
+    // Iterate over all the stack frames and visit objects on the stack.
+    StackFrameIterator frames_iterator(top_exit_frame_info(), validation_policy,
+                                       this, cross_thread_policy);
+    StackFrame* frame = frames_iterator.NextFrame();
+    while (frame != NULL) {
+      frame->VisitObjectPointers(visitor);
+      frame = frames_iterator.NextFrame();
+    }
+  } else {
+    // We are not on the mutator thread.
+    RELEASE_ASSERT(top_exit_frame_info() == 0);
   }
 }
 
diff --git a/runtime/vm/thread_registry.cc b/runtime/vm/thread_registry.cc
index b1bcb2c..bb02973 100644
--- a/runtime/vm/thread_registry.cc
+++ b/runtime/vm/thread_registry.cc
@@ -16,6 +16,10 @@
     MonitorLocker ml(threads_lock());
     // At this point the active list should be empty.
     ASSERT(active_list_ == NULL);
+
+    // The [mutator_thread_] is kept alive until the destruction of the isolate.
+    mutator_thread_->isolate_ = nullptr;
+
     // We have cached the mutator thread, delete it.
     delete mutator_thread_;
     mutator_thread_ = NULL;