diff --git a/library/md.c b/library/md.c
index 8359d05..dfa4526 100644
--- a/library/md.c
+++ b/library/md.c
@@ -39,7 +39,7 @@
 #include "mbedtls/platform.h"
 #else
 #include <stdlib.h>
-#define mbedtls_malloc     malloc
+#define mbedtls_calloc    calloc
 #define mbedtls_free       free
 #endif
 
@@ -216,7 +216,7 @@
 
     if( hmac != 0 )
     {
-        ctx->hmac_ctx = mbedtls_malloc( 2 * md_info->block_size );
+        ctx->hmac_ctx = mbedtls_calloc( 2, md_info->block_size );
         if( ctx->hmac_ctx == NULL )
         {
             md_info->ctx_free_func( ctx->md_ctx );