### The Problem The documentation for `.set` located at https://canjs.com/doc/can-define/map/map.prototype.set.html states: > Assigns value to a property on this map instance called propName. This will define the property if it hasn't already been predefined. This (defining a property, not previously defined) works for plain (not extended) DefineMaps, but fails to work on the example below. ```js const MyMap = DefineMap.extend({}); const mapB = new MyMap({}); mapB.set("propA", "value"); ``` ### The Solution The documentation for `.set` should state (very plainly) that you cannot extend an extended DefineMap and the error it would throw if you try. As a bonus it might be worth explain /why/ an extended DefineMap is sealed where as a plain DefineMap is not.