diff --git a/include/toml11/traits.hpp b/include/toml11/traits.hpp
index 6121f02..11575f4 100644
--- a/include/toml11/traits.hpp
+++ b/include/toml11/traits.hpp
@@ -11,6 +11,7 @@
 #include <string>
 #include <tuple>
 #include <type_traits>
+#include <unordered_set>
 #include <utility>
 
 #if defined(TOML11_HAS_STRING_VIEW)