diff --git a/lib/http_proxy.h b/lib/http_proxy.h
index 5ea0a21af77994a60c8c552391efc52259cf5e14..3caf9287f9df0211aa1c474fc690cfcf0d86ad55 100644
--- a/lib/http_proxy.h
+++ b/lib/http_proxy.h
@@ -1,3 +1,5 @@
+#ifndef HEADER_CURL_HTTP_PROXY_H
+#define HEADER_CURL_HTTP_PROXY_H
 /***************************************************************************
  *                                  _   _ ____  _
  *  Project                     ___| | | |  _ \| |
@@ -31,3 +33,5 @@ CURLcode Curl_proxyCONNECT(struct connectdata *conn,
 #else
 #define Curl_proxyCONNECT(x,y,z,w) CURLE_NOT_BUILT_IN
 #endif
+
+#endif /* HEADER_CURL_HTTP_PROXY_H */