diff --git a/test/test_time_generator_v1_3.cpp b/test/test_time_generator_v1_3.cpp
index 9a5bd3f..80cdd32 100644
--- a/test/test_time_generator_v1_3.cpp
+++ b/test/test_time_generator_v1_3.cpp
@@ -11,6 +11,7 @@
 #include <thread>
 #include <set>
 #include <vector>
+#include <functional> // std::ref
 
 #if defined(BOOST_LIBSTDCXX_VERSION) && BOOST_LIBSTDCXX_VERSION >= 130000
 // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114865
diff --git a/test/test_time_generator_v6_3.cpp b/test/test_time_generator_v6_3.cpp
index 304fedf..58cf7d6 100644
--- a/test/test_time_generator_v6_3.cpp
+++ b/test/test_time_generator_v6_3.cpp
@@ -11,6 +11,7 @@
 #include <thread>
 #include <set>
 #include <vector>
+#include <functional> // std::ref
 
 #if defined(BOOST_LIBSTDCXX_VERSION) && BOOST_LIBSTDCXX_VERSION >= 130000
 // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114865