diff --git a/programs/pkey/rsa_verify.c b/programs/pkey/rsa_verify.c
index f65883d..ccdf13e 100644
--- a/programs/pkey/rsa_verify.c
+++ b/programs/pkey/rsa_verify.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_BIGNUM_C) || !defined(MBEDTLS_RSA_C) ||  \