delete history and freeze transform. It should not matter for mirror but put the pivot on the center line. Also, snap all the verts to the center line and this is IMPORTANT make sure when you snap you are in WORLD mode and keep relative spacing is UNCHECKED.

If you still get this behavior (and I have seen this happen) then you have two options save out a copy of the object and then delete the one in the scene and import or duplicate, scale -1, freeze transforms, reverse normals and then zip the verts manually.