Uncaught TypeError: Cannot call method 'appendChild' of null?

Uncaught TypeError: Cannot call method 'appendChild' of null?

I.m using pagedown editor in my project.
My HTML code:
<div style="padding-top:20px;" class="control-group">
<div class="controls">
<textarea class="wmd-input"
id="wmd-input"></textarea>
</div>
</div>
<div style="padding-top:20px;" class="control-group">
<label for="signature" class="control-label
user-label">ѧУרҵ</label>
<div class="controls">
<div id="wmd-preview" class="wmd-panel
wmd-preview"></div>
</div>
</div>
and my javascript code:
<script type="text/javascript">
$(function()
{
var converter1 = Markdown.getSanitizingConverter();
var editor1 = new Markdown.Editor(converter1);
editor1.run();
});
</script>
google chrome browser throw out an error "Uncaught TypeError: Cannot call
method 'appendChild' of null ?", what's wrong in my code?