Actionscript 3 :: Why Does Delete( DictionaryInstance[ Key ] ); Fail

My app uses a Dictionary

protected _categoryToValueDict:Dictionary = new Dictionary();

to map something to something else.Now, at a certain point in the application, I need to remove a certain key from the Dictionary.I implemented this simple method:

public function setCategoryNoValue( cat:TAModelCategory ):void {
// delete( _categoryToValueDict[ cat ] );
var old:Dictionary = _categoryToValueDict;

[code]....

Thus, my question: What's the difference between the two alternatives? To make the object referenced by myObject eligible for garbage collection, you must remove all references to it. In this case, you must change the value of myObject and delete the myObject key from myMap, as shown in the following code:

myObject = null;
delete myMap[myObject];

Is suppose this to be a typo. Shouldn't it read like this:

delete myMap[myObject];
myObject = null;

Why pass a null-pointer to myMap as key?