Why am I being asked to change the mesh geometry or supply a tensor manually?

I purchased the shopping mall scene from the asset store and used it in my game.

Next I added the cartoon nerd from the asset store to the same scene.

At first the character fell through the shopping mall.

So I added a RigidBody component to the shopping mall game object.

When I try to preview my game in the Unity3D editor, I get the following:
Actor::updateMassFromShapes: Compute mesh inertia tensor failed for one of the actor’s mesh shapes! Please change mesh geometry or supply a tensor manually!

Why?

This happens when you assign a rigidbody to a mesh that is practically volumeless like a plane, etc. To stop your character from going through your shopping mall, just add a collider or multiple colliders to it without a rigidbody.