diff --git a/ext/include/opentelemetry/ext/http/client/curl/http_operation_curl.h b/ext/include/opentelemetry/ext/http/client/curl/http_operation_curl.h
index 55a488f131..65355a05cd 100644
--- a/ext/include/opentelemetry/ext/http/client/curl/http_operation_curl.h
+++ b/ext/include/opentelemetry/ext/http/client/curl/http_operation_curl.h
@@ -13,6 +13,7 @@
 #  include <future>
 #endif
 
+#include <functional>
 #include <map>
 #include <regex>
 #include <sstream>