diff --git a/programs/pkey/pk_sign.c b/programs/pkey/pk_sign.c
index 2e9b2d8..82cb6a1 100644
--- a/programs/pkey/pk_sign.c
+++ b/programs/pkey/pk_sign.c
@@ -20,7 +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.
+/* 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_ENTROPY_C) ||  \