diff --git a/library/x509.c b/library/x509.c
index fe4e3e3..54275eb 100644
--- a/library/x509.c
+++ b/library/x509.c
@@ -314,7 +314,7 @@
         /* Only MFG1 is recognised for now */
         if (MBEDTLS_OID_CMP(MBEDTLS_OID_MGF1, &alg_id) != 0) {
             return MBEDTLS_ERROR_ADD(MBEDTLS_ERR_X509_FEATURE_UNAVAILABLE,
-                                     MBEDTLS_ERR_OID_NOT_FOUND);
+                                     MBEDTLS_ERR_X509_UNKNOWN_OID);
         }
 
         /* Parse HashAlgorithm */