diff --git a/src/blockstore/implementations/versioncounting/KnownBlockVersions.cpp b/src/blockstore/implementations/versioncounting/KnownBlockVersions.cpp
index c810a0c8..18686995 100644
--- a/src/blockstore/implementations/versioncounting/KnownBlockVersions.cpp
+++ b/src/blockstore/implementations/versioncounting/KnownBlockVersions.cpp
@@ -19,6 +19,7 @@ namespace blockstore {
 namespace versioncounting {
 
 const string KnownBlockVersions::HEADER = "cryfs.integritydata.knownblockversions;0";
+constexpr uint32_t KnownBlockVersions::CLIENT_ID_FOR_DELETED_BLOCK;
 
 KnownBlockVersions::KnownBlockVersions(const bf::path &stateFilePath)
         :_knownVersions(), _lastUpdateClientId(), _stateFilePath(stateFilePath), _myClientId(0), _mutex(), _valid(true) {
@@ -46,6 +47,7 @@ KnownBlockVersions::~KnownBlockVersions() {
 
 bool KnownBlockVersions::checkAndUpdateVersion(uint32_t clientId, const Key &key, uint64_t version) {
     unique_lock<mutex> lock(_mutex);
+    ASSERT(clientId != CLIENT_ID_FOR_DELETED_BLOCK, "This is not a valid client id");
 
     ASSERT(version > 0, "Version has to be >0"); // Otherwise we wouldn't handle notexisting entries correctly.
     ASSERT(_valid, "Object not valid due to a std::move");
@@ -85,7 +87,9 @@ void KnownBlockVersions::_loadStateFile() {
     optional<Data> file = Data::LoadFromFile(_stateFilePath);
     if (file == none) {
         // File doesn't exist means we loaded empty state. Assign a random client id.
-        _myClientId = *reinterpret_cast<uint32_t*>(Random::PseudoRandom().getFixedSize<sizeof(uint32_t)>().data());
+        do {
+            _myClientId = *reinterpret_cast<uint32_t*>(Random::PseudoRandom().getFixedSize<sizeof(uint32_t)>().data());
+        } while(_myClientId == CLIENT_ID_FOR_DELETED_BLOCK); // Safety check - CLIENT_ID_FOR_DELETED_BLOCK shouldn't be used by any valid client.
         return;
     }
 
@@ -183,8 +187,13 @@ uint32_t KnownBlockVersions::myClientId() const {
 }
 
 uint64_t KnownBlockVersions::getBlockVersion(uint32_t clientId, const Key &key) const {
+    unique_lock<mutex> lock(_mutex);
     return _knownVersions.at({clientId, key});
 }
 
+void KnownBlockVersions::markBlockAsDeleted(const Key &key) {
+    _lastUpdateClientId[key] = CLIENT_ID_FOR_DELETED_BLOCK;
+}
+
 }
 }
diff --git a/src/blockstore/implementations/versioncounting/KnownBlockVersions.h b/src/blockstore/implementations/versioncounting/KnownBlockVersions.h
index 795bda8a..f4b24b60 100644
--- a/src/blockstore/implementations/versioncounting/KnownBlockVersions.h
+++ b/src/blockstore/implementations/versioncounting/KnownBlockVersions.h
@@ -25,6 +25,8 @@ namespace blockstore {
 
             uint64_t incrementVersion(const Key &key, uint64_t lastVersion);
 
+            void markBlockAsDeleted(const Key &key);
+
             uint64_t getBlockVersion(uint32_t clientId, const Key &key) const;
 
             uint32_t myClientId() const;
@@ -38,6 +40,8 @@ namespace blockstore {
             mutable std::mutex _mutex;
             bool _valid;
 
+            static constexpr uint32_t CLIENT_ID_FOR_DELETED_BLOCK = 0;
+
             static const std::string HEADER;
 
             void _loadStateFile();
diff --git a/src/blockstore/implementations/versioncounting/VersionCountingBlock.h b/src/blockstore/implementations/versioncounting/VersionCountingBlock.h
index 531d8d9b..49264a3d 100644
--- a/src/blockstore/implementations/versioncounting/VersionCountingBlock.h
+++ b/src/blockstore/implementations/versioncounting/VersionCountingBlock.h
@@ -41,7 +41,6 @@ public:
   size_t size() const override;
   void resize(size_t newSize) override;
 
-  uint64_t version() const;
   cpputils::unique_ref<Block> releaseBlock();
 
 #ifndef CRYFS_NO_COMPATIBILITY
@@ -208,10 +207,6 @@ inline uint64_t VersionCountingBlock::blockSizeFromPhysicalBlockSize(uint64_t bl
   return blockSize - HEADER_LENGTH;
 }
 
-inline uint64_t VersionCountingBlock::version() const {
-  return _version;
-}
-
 }
 }
 
diff --git a/src/blockstore/implementations/versioncounting/VersionCountingBlockStore.h b/src/blockstore/implementations/versioncounting/VersionCountingBlockStore.h
index f9c1cba1..82fb1961 100644
--- a/src/blockstore/implementations/versioncounting/VersionCountingBlockStore.h
+++ b/src/blockstore/implementations/versioncounting/VersionCountingBlockStore.h
@@ -66,7 +66,7 @@ inline void VersionCountingBlockStore::remove(cpputils::unique_ref<Block> block)
   Key key = block->key();
   auto versionCountingBlock = cpputils::dynamic_pointer_move<VersionCountingBlock>(block);
   ASSERT(versionCountingBlock != boost::none, "Block is not an VersionCountingBlock");
-  _knownBlockVersions.incrementVersion(key, (*versionCountingBlock)->version());
+  _knownBlockVersions.markBlockAsDeleted(key);
   auto baseBlock = (*versionCountingBlock)->releaseBlock();
   _baseBlockStore->remove(std::move(baseBlock));
 }