diff --git a/programs/pkey/dh_server.c b/programs/pkey/dh_server.c
index 54165dc..b883653 100644
--- a/programs/pkey/dh_server.c
+++ b/programs/pkey/dh_server.c
@@ -20,6 +20,7 @@
 #include "mbedtls/build_info.h"
 
 #include "mbedtls/platform.h"
+// md.h is included this early since MD_CAN_XXX macros are defined there.
 #include "mbedtls/md.h"
 
 #if defined(MBEDTLS_AES_C) && defined(MBEDTLS_DHM_C) && \