diff --git a/proxy_wasm_api.h b/proxy_wasm_api.h
index 166b49c..a97503e 100644
--- a/proxy_wasm_api.h
+++ b/proxy_wasm_api.h
@@ -28,6 +28,7 @@
 #include <cstring>
 #include <functional>
 #include <memory>
+#include <optional>
 #include <string>
 #include <tuple>
 #include <unordered_map>