diff --git a/library/x509write_crt.c b/library/x509write_crt.c
index e8d5cbd..0af23d7 100644
--- a/library/x509write_crt.c
+++ b/library/x509write_crt.c
@@ -321,7 +321,7 @@
     else if( mbedtls_pk_can_do( ctx->issuer_key, MBEDTLS_PK_ECDSA ) )
         pk_alg = MBEDTLS_PK_ECDSA;
     else
-        pk_alg = MBEDTLS_PK_NONE;
+        return( MBEDTLS_ERR_X509_INVALID_ALG );
 
     if( ( ret = mbedtls_oid_get_oid_by_sig_alg( pk_alg, ctx->md_alg,
                                           &sig_oid, &sig_oid_len ) ) != 0 )