Decimate modifier

Hallo everyone!

I’ve run into problems when I try to add an decimate modifier:

It says: non-manifold mesh as input. Does anyone know how to fix it?

And is threre a better way to reduce polygons, I know you can you use retopo an baking the normals.

Thanks in advance!

http://www.blender.org/forum/viewtopic.php?t=1806&sid=d7c999a0670cdb55c7121fd4523f475e