diff --git a/src/opustags.h b/src/opustags.h
index 438e604..f9d9240 100644
--- a/src/opustags.h
+++ b/src/opustags.h
@@ -31,6 +31,7 @@
 #include <functional>
 #include <list>
 #include <memory>
+#include <optional>
 #include <string>
 #include <vector>