pgrouting - pgr_dijkstra working but pgr_ksp but returning anything for the same params

The following returns a route as expected.

SELECT seq, id1 AS node, id2 AS edge, cost, b.the_geom as the_geom FROM pgr_dijkstra('

SELECT gid AS id,
     source::integer,
     target::integer,
     length::double precision AS cost
    FROM ways',
247542, 258349, false, false) a LEFT JOIN ways b ON (a.id2 = b.gid);

However the following does not. I expected this to return 3 routes but instead it returns nothing.

SELECT seq, id1 AS node, id2 AS edge, cost, b.the_geom as the_geom FROM pgr_ksp('
SELECT gid AS id,

     source::integer,
     target::integer,
     length::double precision AS cost
    FROM ways',
247542, 258349, 3, false) a LEFT JOIN ways b ON (a.id2 = b.gid);

Any idea why this is happening?