MeshRenderer override materials at run-time.

I'm having an issue with over-ridding materials at run-time in 5r18.

Unable to override materials using .materials[] or .shaderMaterials[].
When I try to override a material in slot 0/1/2/3... nothing happens.
mesh_renderer.materials[ slot ] = the runtime material;
or
mesh_renderer.sharedMaterials[ slot ] = the runtime material;
I've tried executing this in an attached component in Awake, Start and the first call to Update- none actually change the material on the object.
Using mesh_renderer.material = the runtime material; works correctly.

This code was previously valid in Unity 4, and I don't see any obvious work around's in MeshRenderer.