diff --git a/library/x509.c b/library/x509.c
index c1fd8ad..33b1ac3 100644
--- a/library/x509.c
+++ b/library/x509.c
@@ -269,7 +269,7 @@
             return( ret );
 
         /* Only MFG1 is recognised for now */
-        if( ! OID_CMP( OID_MGF1, &alg_id ) )
+        if( OID_CMP( OID_MGF1, &alg_id ) != 0 )
             return( POLARSSL_ERR_X509_FEATURE_UNAVAILABLE +
                     POLARSSL_ERR_OID_NOT_FOUND );