The proposed patch would potentially break existing code which anticipates the current behavior.  The potential negative impact means this particular proposed patch is not viable.

Choices forward include:  (1) better documenting the existing, established implementation behavior (perhaps having the assert provide a more informative message), or (2) altering the behavior in a new release version (perhaps 3.6 when it comes) and documenting that change appropriately there.

As to what sort of exception we should expect in this situation, unfortunately comparing to the queue module's Queue does not help guide expectations much since it does not have need of a close().

Of the two above options, I'm more inclined towards the first.  Does @Joseph.Siddall have other motivations that helped motivate this suggested improvement?