diff --git a/src/microhttpd/connection.c b/src/microhttpd/connection.c
index 8ba793f4..c86dadf5 100644
--- a/src/microhttpd/connection.c
+++ b/src/microhttpd/connection.c
@@ -54,6 +54,7 @@
 #include <sys/param.h>
 #endif /* HAVE_SYS_PARAM_H */
 #include "mhd_send.h"
+#include "mhd_assert.h"
 
 /**
  * Message to transmit when http 1.1 request is received
diff --git a/src/microhttpd/mhd_assert.h b/src/microhttpd/mhd_assert.h
index e99632d0..427070eb 100644
--- a/src/microhttpd/mhd_assert.h
+++ b/src/microhttpd/mhd_assert.h
@@ -1,6 +1,6 @@
 /*
   This file is part of libmicrohttpd
-  Copyright (C) 2017 Karlson2k (Evgeny Grin)
+  Copyright (C) 2017-2021 Karlson2k (Evgeny Grin)
 
   This library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
@@ -23,10 +23,17 @@
  * @author Karlson2k (Evgeny Grin)
  */
 
+/* Unlike POSIX version of 'assert.h', MHD version of 'assert' header
+ * does not allow multiple redefinition of 'mhd_assert' macro within single
+ * souce file. */
 #ifndef MHD_ASSERT_H
 #define MHD_ASSERT_H 1
 
 #include "mhd_options.h"
+
+#if ! deifned(_DEBUG) && ! defined(NDEBUG)
+#define NDEBUG 1 /* Use NDEBUG by default */
+#endif /* !_DEBUG && !NDEBUG */
 #ifdef NDEBUG
 #  define mhd_assert(ignore) ((void) 0)
 #else  /* _DEBUG */