diff --git a/implementations/caching/cache/QueueMap.h b/implementations/caching/cache/QueueMap.h
index 6d36a13c..1f3fd56d 100644
--- a/implementations/caching/cache/QueueMap.h
+++ b/implementations/caching/cache/QueueMap.h
@@ -26,7 +26,9 @@ public:
 
   void push(const Key &key, Value value) {
     auto newEntry = _entries.emplace(std::piecewise_construct, std::forward_as_tuple(key), std::forward_as_tuple(_sentinel.prev, &_sentinel));
-    ASSERT(newEntry.second == true, "There is already an element with this key");
+    if (!newEntry.second) {
+      throw std::logic_error("There is already an element with this key");
+    }
     newEntry.first->second.init(&newEntry.first->first, std::move(value));
     //The following is ok, because std::unordered_map never invalidates pointers to its entries
     _sentinel.prev->next = &newEntry.first->second;
diff --git a/implementations/ondisk/OnDiskBlockStore.cpp b/implementations/ondisk/OnDiskBlockStore.cpp
index 53adff6a..a6e32168 100644
--- a/implementations/ondisk/OnDiskBlockStore.cpp
+++ b/implementations/ondisk/OnDiskBlockStore.cpp
@@ -15,6 +15,8 @@ namespace ondisk {
 OnDiskBlockStore::OnDiskBlockStore(const boost::filesystem::path &rootdir)
  : _rootdir(rootdir) {}
 
+//TODO Do I have to lock tryCreate/remove and/or load? Or does ParallelAccessBlockStore take care of that?
+
 optional<unique_ref<Block>> OnDiskBlockStore::tryCreate(const Key &key, Data data) {
   //TODO Easier implementation? This is only so complicated because of the cast OnDiskBlock -> Block
   auto result = std::move(OnDiskBlock::CreateOnDisk(_rootdir, key, std::move(data)));
diff --git a/implementations/testfake/FakeBlockStore.cpp b/implementations/testfake/FakeBlockStore.cpp
index e03f7754..89604f96 100644
--- a/implementations/testfake/FakeBlockStore.cpp
+++ b/implementations/testfake/FakeBlockStore.cpp
@@ -19,6 +19,7 @@ FakeBlockStore::FakeBlockStore()
  : _blocks(), _used_dataregions_for_blocks() {}
 
 optional<unique_ref<Block>> FakeBlockStore::tryCreate(const Key &key, Data data) {
+  std::unique_lock<std::mutex> lock(_mutex);
   auto insert_result = _blocks.emplace(key.ToString(), std::move(data));
 
   if (!insert_result.second) {
@@ -26,10 +27,15 @@ optional<unique_ref<Block>> FakeBlockStore::tryCreate(const Key &key, Data data)
   }
 
   //Return a copy of the stored data
-  return load(key);
+  return _load(key);
 }
 
 optional<unique_ref<Block>> FakeBlockStore::load(const Key &key) {
+  std::unique_lock<std::mutex> lock(_mutex);
+  return _load(key);
+}
+
+optional<unique_ref<Block>> FakeBlockStore::_load(const Key &key) {
   //Return a copy of the stored data
   string key_string = key.ToString();
   try {
@@ -42,6 +48,7 @@ optional<unique_ref<Block>> FakeBlockStore::load(const Key &key) {
 void FakeBlockStore::remove(unique_ref<Block> block) {
   Key key = block->key();
   cpputils::destruct(std::move(block));
+  std::unique_lock<std::mutex> lock(_mutex);
   int numRemoved = _blocks.erase(key.ToString());
   ASSERT(numRemoved == 1, "Block not found");
 }
@@ -53,6 +60,7 @@ unique_ref<Block> FakeBlockStore::makeFakeBlockFromData(const Key &key, const Da
 }
 
 void FakeBlockStore::updateData(const Key &key, const Data &data) {
+  std::unique_lock<std::mutex> lock(_mutex);
   auto found = _blocks.find(key.ToString());
   if (found == _blocks.end()) {
     auto insertResult = _blocks.emplace(key.ToString(), data.copy());
@@ -65,6 +73,7 @@ void FakeBlockStore::updateData(const Key &key, const Data &data) {
 }
 
 uint64_t FakeBlockStore::numBlocks() const {
+  std::unique_lock<std::mutex> lock(_mutex);
   return _blocks.size();
 }
 
diff --git a/implementations/testfake/FakeBlockStore.h b/implementations/testfake/FakeBlockStore.h
index fc04696b..b0627d4d 100644
--- a/implementations/testfake/FakeBlockStore.h
+++ b/implementations/testfake/FakeBlockStore.h
@@ -48,7 +48,10 @@ private:
   //We want to avoid this for the reasons mentioned above (overflow data).
   std::vector<std::shared_ptr<cpputils::Data>> _used_dataregions_for_blocks;
 
+  mutable std::mutex _mutex;
+
   cpputils::unique_ref<Block> makeFakeBlockFromData(const Key &key, const cpputils::Data &data, bool dirty);
+  boost::optional<cpputils::unique_ref<Block>> _load(const Key &key);
 
   DISALLOW_COPY_AND_ASSIGN(FakeBlockStore);
 };