diff --git a/test/t/index/test_dump_and_load_index.cpp b/test/t/index/test_dump_and_load_index.cpp
index 3d0c0f1f..be855069 100644
--- a/test/t/index/test_dump_and_load_index.cpp
+++ b/test/t/index/test_dump_and_load_index.cpp
@@ -16,7 +16,7 @@ using sparse_file_array = osmium::index::map::SparseFileArray<osmium::unsigned_o
 namespace {
 
 template <class TMemoryIndex, class TFileIndex>
-void test_index(std::function<void(TMemoryIndex&, const int)> dump_method) {
+void test_index(const std::function<void(TMemoryIndex&, const int)>& dump_method) {
     const int fd = osmium::detail::create_tmp_file();
     REQUIRE(osmium::file_size(fd) == 0);
     const osmium::unsigned_object_id_type id1 = 12;
diff --git a/test/t/tags/test_filter.cpp b/test/t/tags/test_filter.cpp
index 8753a93d..15007d66 100644
--- a/test/t/tags/test_filter.cpp
+++ b/test/t/tags/test_filter.cpp
@@ -18,7 +18,7 @@ namespace {
 
 template <class TFilter>
 void check_filter(const osmium::TagList& tag_list,
-                  const TFilter filter,
+                  const TFilter& filter,
                   const std::vector<bool>& reference) {
     REQUIRE(tag_list.size() == reference.size());
     auto t_it = tag_list.begin();