Nonmanifold edge .. boolean Error

anyone know this Error?

here is that Error 3DM file

Isolate the red arrow, then explode it.
You will find out there are two surfaces at the bottom of the arrow.
Clean that and the boolean difference will work.

Thanks,
-C-H-A-R-L-E-S-

Nonmanifold edges are described and shown in the Help file.