Why does currentNode.getNodes() not return a NodeIterator?

[Clemens Wyss]

my esp script starts as follows:
if (currentNode)
{
  var nodes = currentNode.getNodes( );

  var size = nodes.getSize();
...

I keep getting:
Uncaught SlingException org.mozilla.javascript.EcmaError: TypeError: getSize is 
not a function, it is org.mozilla.javascript.Undefined

what am I missing? I know I shouldn't be usign currentNode at all, but after 
all currentNode is not null and the doc says that getNodes() always returns a 
NodeIterator...