diff --git a/programs/x509/cert_req.c b/programs/x509/cert_req.c
index e651f3c..f35d629 100644
--- a/programs/x509/cert_req.c
+++ b/programs/x509/cert_req.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_CSR_WRITE_C) || !defined(MBEDTLS_FS_IO) ||  \