diff --git a/mycpp/gc_dict.h b/mycpp/gc_dict.h
index 4b04c6a67a..a62dc68b88 100644
--- a/mycpp/gc_dict.h
+++ b/mycpp/gc_dict.h
@@ -48,7 +48,7 @@ class _DummyDict {
   void* values_;
 };
 
-// A list has one Slab pointer which we need to follow.
+// A dict has 3 pointers the GC needs to follow.
 constexpr uint16_t maskof_Dict() {
   return maskbit(offsetof(_DummyDict, entry_)) |
          maskbit(offsetof(_DummyDict, keys_)) |
diff --git a/mycpp/marksweep_gc_test.cc b/mycpp/marksweep_gc_test.cc
index 2bb6157abd..267bbcf908 100644
--- a/mycpp/marksweep_gc_test.cc
+++ b/mycpp/marksweep_gc_test.cc
@@ -82,6 +82,40 @@ TEST list_collection_test() {
   PASS();
 }
 
+class Node : Obj {
+ public:
+  Node();
+  Node* next_;
+};
+
+constexpr uint16_t maskof_Node() {
+  return maskbit(offsetof(Node, next_));
+}
+
+Node::Node() :
+  Obj(Tag::FixedSize, maskof_Node(), sizeof(Node)),
+  next_(nullptr) {
+}
+
+TEST cycle_collection_test() {
+  // Dict<Str*, int>* d = NewDict<Str*, int>();
+
+  Node* n1 = nullptr;
+  Node* n2 = nullptr;
+  StackRoots _roots({&n1, &n2});
+  n1 = Alloc<Node>();
+  n2 = Alloc<Node>();
+
+  gHeap.Collect();
+
+  n1->next_ = n2;
+  n2->next_ = n1;
+
+  gHeap.Collect();
+
+  PASS();
+}
+
 GREATEST_MAIN_DEFS();
 
 int main(int argc, char **argv) {
@@ -93,6 +127,9 @@ int main(int argc, char **argv) {
 
   RUN_TEST(string_collection_test);
   RUN_TEST(list_collection_test);
+  RUN_TEST(cycle_collection_test);
+
+  gHeap.Collect();
 
   GREATEST_MAIN_END(); /* display results */
   return 0;
diff --git a/mycpp/marksweep_heap.cc b/mycpp/marksweep_heap.cc
index 316e5a835f..d28b89d113 100644
--- a/mycpp/marksweep_heap.cc
+++ b/mycpp/marksweep_heap.cc
@@ -53,6 +53,13 @@ void* MarkSweepHeap::Allocate(int byte_count) {
 void MarkSweepHeap::MarkAllReferences(Obj* obj) {
   auto header = ObjHeader(obj);
 
+  auto marked_alloc = marked_allocations_.find((Obj*)obj);
+  bool alloc_is_marked = marked_alloc != marked_allocations_.end();
+  if (alloc_is_marked)
+  {
+    return;
+  }
+
   this->marked_allocations_.insert(static_cast<void*>(obj));
 
   switch (header->heap_tag_) {