Convex mesh collider thinks model is non manifold but it isn't

I'm getting this error in unity on my convex mesh collider.

Adjacencies::CreateDatabase: can't work on non-manifold meshes. Try to use the PxConvexFlag::eINFLATE_CONVEX flag (see PxToolkit::createConvexMeshSafe) UnityEditor.DockArea:OnGUI() GU::ConvexMesh::loadConvexHull: convex hull init failed! Try to use the PxConvexFlag::eINFLATE_CONVEX flag. (see PxTooklit::createConvexMeshSafe) UnityEditor.DockArea:OnGUI()

The thing is my mesh isn't non manifold. It has less than 255 triangles. I've used blender's Select non-manifold and it comes up with nothing. I've tried deleting doubles but there aren't any. I have no idea what's wrong. I have another mesh that is very similar and when I use it as a mesh collider I have no problems. The mesh is a .blend file. Here is an image of the mesh:

Thanks.