diff --git a/programs/x509/cert_write.c b/programs/x509/cert_write.c
index 9ff9488..5c911ca 100644
--- a/programs/x509/cert_write.c
+++ b/programs/x509/cert_write.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_X509_CRT_WRITE_C) || \