diff --git a/tests/notnull_tests.cpp b/tests/notnull_tests.cpp
index c3fb388..fd6e6c0 100644
--- a/tests/notnull_tests.cpp
+++ b/tests/notnull_tests.cpp
@@ -24,6 +24,7 @@
 #include <stdint.h>  // for uint16_t
 #include <string>    // for basic_string, operator==, string, operator<<
 #include <typeinfo>  // for type_info
+#include <functional>
 
 namespace gsl {
 struct fail_fast;