Dragging Lights to Scene Tree Buggy

[Ed]

When I drag one of the pre-defined lights from the Material Library to an object in the Scene Tree, it comes across initially OK.  When I then drag a different light onto the same object in the tree, the original light remains the majority of the time.  In other words, once a light is assigned to an object in the Scene Tree, it doesn't allow a new assignment (light or non-light material) the majority of the time.  It doesn't matter what kind of light or other material I try.

My non-lighting material assignments/ re-assignments work fine when dragging to the Scene Tree.

WIN8 64 bit  KS ver 4.1.35

Ed