Adding property with custom namespace to dynamic object

dynamicObjectTest.as

package 
{
	public function dynamicObjectTest():void 
	{
		// Namespace works elsewhere, but not on this dynamic object
		//Exception fault: ReferenceError: Error #1056: Cannot create property custom::prop on Object.
		var obj:Object = {};
		obj.custom::prop = "val";
		
		for (var property:* in obj)
		{
			trace(property, obj[property]);
		}
	}
}