Actionscript 3 :: Instanceof For Dynamic Interface?

interface If { ... }
class Impl implements If { ... }
function test(type:Class, obj) {
return obj instanceof type;
}
test(If, new Impl());

The call to test on the last line returns false, but it should be true. How can I do this check right, with the requirement that it must be inside the function?

Real code:

public function iterate(callback:Function, type:Class = null) {
for (var node:EntityListNode = beginNode; node != null; node = node.next) {
if (type == null || node.entity instanceof type) callback(node.entity);
}
}