Call to drupal_render() returns NULL

I have been looking since yesterday but can't understand why a call to drupal_render() is returning NULL. In my node theme function, when I call drupal_render($element) wi开发者_如何学JAVAth $element being defined like below, it renders nothing!

<?php
Array
(
    [#type] => textfield
    [#title] => Label
    [#default_value] => 
    [#post] => Array
        (
        )

    [#programmed] => 
    [#tree] => 
    [#parents] => Array
        (
            [0] => label
        )

    [#array_parents] => Array
        (
            [0] => range
            [1] => label
        )

    [#weight] => 0
    [#processed] => 1
    [#description] => 
    [#attributes] => Array
        (
        )

    [#required] => 
    [#input] => 1
    [#size] => 60
    [#maxlength] => 128
    [#autocomplete_path] => 
    [#process] => Array
        (
            [0] => form_expand_ahah
        )

    [#name] => label
    [#id] => edit-label
    [#value] => 
    [#defaults_loaded] => 1
    [#sorted] => 1
    [#printed] => 1
)
?>

Any idea what I am missing?

Check drupal source for this function:

if (!isset($elements) || (isset($elements['#access']) && !$elements['#access'])) {
    return NULL;
  }

You see now how it can return null?