--- a/mac.c
+++ b/mac.c
@@ -142,6 +142,7 @@
 		/* reset HMAC context */
 		HMAC_Init(&mac->evp_ctx, NULL, 0, NULL);
 		HMAC_Update(&mac->evp_ctx, b, sizeof(b));
+		HMAC_CTX_set_flags(&mac->evp_ctx, EVP_MD_CTX_FLAG_ONESHOT);
 		HMAC_Update(&mac->evp_ctx, data, datalen);
 		HMAC_Final(&mac->evp_ctx, m, NULL);
 		break;