threecsg.js and subdivision modifier distorted geometry

Question

I have the latest threecsg.js library and my use of it has been okay, except when I try to use the subdivision modifier after a CSG operation. Here is example code, modified from the example.html file that comes with the library from github:

var start_time = (new Date()).getTime();
var cube_geometry = new THREE.CubeGeometry( 3, 3, 3 );
var cube_mesh = new THREE.Mesh( cube_geometry );
cube_mesh.position.x = -6;
var cube_bsp = new ThreeBSP( cube_mesh );

var sphere_geometry = new THREE.SphereGeometry( 1.8, 12, 12 );
var sphere_mesh = new THREE.Mesh( sphere_geometry );
sphere_mesh.position.x = -7;
sphere_mesh.position.y -= 0;
var sphere_bsp = new ThreeBSP( sphere_mesh );

var subtract_bsp = cube_bsp.union( sphere_bsp );
var result = subtract_bsp.toMesh( new THREE.MeshLambertMaterial({ shading: THREE.SmoothShading, map: THREE.ImageUtils.loadTexture('texture.png') }) );
result.geometry.computeVertexNormals();

var smooth = result.geometry.clone() ;

smooth.mergeVertices();

var modifier = new THREE.SubdivisionModifier(0.1);

modifier.modify( smooth );

var mesh = new THREE.Mesh( smooth, new THREE.MeshPhongMaterial( { wireframe:true, color: 0xffffff } ) );
mesh.geometry.computeFaceNormals();
scene.add( mesh );

The above code unites a sphere and a cube. After this, it runs the resulting geometry through the subdivision modifier. The final output that is added to the scene has faces that are protruding from the object, other than that, the object does look smooth. Can anyone please help in solving this issue, that is, removing the protruding faces?