diff --git a/README.md b/README.md
index e61d90f8..32ffdae2 100644
--- a/README.md
+++ b/README.md
@@ -67,4 +67,4 @@ See [INSTALL.md](INSTALL.md) file.
 
 ## Contributing
 
-Contributions are welcome! Please prepare your pull requests on the [develop](../../tree/develop) branch.
+Contributions are welcome! Please submit your pull requests against the [develop](../../tree/develop) branch.
diff --git a/src/libs/database/impl/Cluster.cpp b/src/libs/database/impl/Cluster.cpp
index 26dd4b21..0a3143c8 100644
--- a/src/libs/database/impl/Cluster.cpp
+++ b/src/libs/database/impl/Cluster.cpp
@@ -132,11 +132,25 @@ ClusterType::getAllOrphans(Session& session)
 {
 	session.checkSharedLocked();
 
-	Wt::Dbo::collection<pointer> res = session.getDboSession().query<Wt::Dbo::ptr<ClusterType>>("select c_t from cluster_type c_t LEFT OUTER JOIN cluster c ON c_t.id = c.cluster_type_id WHERE c.id IS NULL");
+	Wt::Dbo::collection<pointer> res = session.getDboSession().query<Wt::Dbo::ptr<ClusterType>>(
+			"SELECT c_t from cluster_type c_t"
+			" LEFT OUTER JOIN cluster c ON c_t.id = c.cluster_type_id")
+		.where("c.id IS NULL");
 
 	return std::vector<pointer>(res.begin(), res.end());
 }
 
+std::vector<ClusterType::pointer>
+ClusterType::getAllUsed(Session& session)
+{
+	session.checkSharedLocked();
+
+	Wt::Dbo::collection<pointer> res = session.getDboSession().query<Wt::Dbo::ptr<ClusterType>>(
+			"SELECT DISTINCT c_t from cluster_type c_t")
+		.join("cluster c ON c_t.id = c.cluster_type_id");
+
+	return std::vector<pointer>(res.begin(), res.end());
+}
 
 ClusterType::pointer
 ClusterType::getByName(Session& session, const std::string& name)
diff --git a/src/libs/database/include/database/Cluster.hpp b/src/libs/database/include/database/Cluster.hpp
index fb0e4518..71e0da7f 100644
--- a/src/libs/database/include/database/Cluster.hpp
+++ b/src/libs/database/include/database/Cluster.hpp
@@ -92,6 +92,7 @@ class ClusterType : public Wt::Dbo::Dbo<ClusterType>
 		ClusterType(std::string name);
 
 		static std::vector<pointer> getAllOrphans(Session& session);
+		static std::vector<pointer> getAllUsed(Session& session);
 		static pointer getByName(Session& session, const std::string& name);
 		static pointer getById(Session& session, IdType id);
 		static std::vector<pointer> getAll(Session& session);
diff --git a/src/lms/ui/explore/Filters.cpp b/src/lms/ui/explore/Filters.cpp
index e893ba40..06c34da9 100644
--- a/src/lms/ui/explore/Filters.cpp
+++ b/src/lms/ui/explore/Filters.cpp
@@ -52,7 +52,7 @@ Filters::showDialog()
 	{
 		auto transaction {LmsApp->getDbSession().createSharedTransaction()};
 
-		const auto types {Database::ClusterType::getAll(LmsApp->getDbSession())};
+		const auto types {Database::ClusterType::getAllUsed(LmsApp->getDbSession())};
 		for (const Database::ClusterType::pointer& type : types)
 			typeCombo->addItem(Wt::WString::fromUTF8(type->getName()));
 
diff --git a/src/test/database/DatabaseTest.cpp b/src/test/database/DatabaseTest.cpp
index d12e3c52..6569f1a4 100644
--- a/src/test/database/DatabaseTest.cpp
+++ b/src/test/database/DatabaseTest.cpp
@@ -228,7 +228,6 @@ testSingleCluster(Session& session)
 		{
 			auto transaction {session.createUniqueTransaction()};
 
-
 			auto clusters {Cluster::getAll(session)};
 			CHECK(clusters.size() == 1);
 			CHECK(clusters.front().id() == cluster.getId());
@@ -242,6 +241,10 @@ testSingleCluster(Session& session)
 			CHECK(clusterTypes.size() == 1);
 			CHECK(clusterTypes.front().id() == clusterType.getId());
 
+			clusterTypes = ClusterType::getAllUsed(session);
+			CHECK(clusterTypes.size() == 1);
+			CHECK(clusterTypes.front().id() == clusterType.getId());
+
 			clusterTypes = ClusterType::getAllOrphans(session);
 			CHECK(clusterTypes.empty());
 		}
@@ -253,6 +256,8 @@ testSingleCluster(Session& session)
 		auto clusterTypes {ClusterType::getAllOrphans(session)};
 		CHECK(clusterTypes.size() == 1);
 		CHECK(clusterTypes.front().id() == clusterType.getId());
+
+		CHECK(ClusterType::getAllUsed(session).empty());
 	}
 }