Bug in BRepAlgoAPI_Fuse?

I'm trying to build a model by fusing together a number of cylindrical tubes, with spheres to fill in the gaps at the junctions. The file at http://www.ceiriog.eclipse.co.uk/test-fuse.cc shows the problem. It tries to create a closed loop consisting of cylindrical tubes. But when NCYLS is even, it fails with the error message

Fuse error status 101
terminate called after throwing an instance of 'StdFail_NotDone'
Aborted

When NCYLS is odd, it works OK. It this a bug?