diff --git a/WORKSPACE b/WORKSPACE
index 5592972..adbc458 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -30,7 +30,6 @@ load("@bazel_skylib//:workspace.bzl", "bazel_skylib_workspace")
 
 bazel_skylib_workspace()
 
-
 git_repository(
     name = "com_google_protobuf",
     remote = "https://github.com/protocolbuffers/protobuf",
diff --git a/proxy_wasm_api.h b/proxy_wasm_api.h
index f05eca7..05efc86 100644
--- a/proxy_wasm_api.h
+++ b/proxy_wasm_api.h
@@ -19,6 +19,7 @@
  * Intrinsic high-level support functions available to WASM modules.
  */
 // NOLINT(namespace-envoy)
+#include <functional>
 #include <string>
 #include <tuple>
 #include <unordered_map>