mongodb c-driver $orderby in queries

I'm trying to make a query to mongo collection using sort ($orderby) keyword.

Here is bson-query object without $orderby:

bson cmd;
bson_init( &cmd );
bson_append_string( &cmd, "ip", ip.c_str() );
bson_append_start_object( &cmd, "timestamp" );
bson_append_long( &cmd, "$gt", 100 );
bson_append_finish_object( &cmd );
bson_finish( &cmd );

mongo_find( conn, collection, cmd, bson_empty( &empty ), 0, 0, MONGO_SLAVE_OK );

It works great and returnes 5000 results.

When I add $orderby object (like in c-driver example here http://api.mongodb.org/c/current/tutorial.html#complex-queries):

bson cmd;
bson_init( &cmd );
bson_append_string( &cmd, "ip", ip.c_str() );
bson_append_start_object( &cmd, "timestamp" );
bson_append_long( &cmd, "$gt", 100 );
bson_append_finish_object( &cmd );

bson_append_start_object( &cmd, "$orderby" );
bson_append_int( &cmd, "timestamp", 1 );
bson_append_finish_object( &cmd );

bson_finish( &cmd );
...

I have 0 results.

What I do wrong?

Try:

bson cmd;
bson_init( &cmd );

bson_append_start_object( &cmd, "$query" );
    bson_append_string( &cmd, "ip", ip.c_str() );
    bson_append_start_object( &cmd, "timestamp" );
        bson_append_long( &cmd, "$gt", 100 );
    bson_append_finish_object( &cmd );
bson_append_finish_object( &cmd );

bson_append_start_object( &cmd, "$orderby" );
    bson_append_int( &cmd, "timestamp", 1 );
bson_append_finish_object( &cmd );

bson_finish( &cmd );