Selenium command isEditable doesn't seem to work

Hi,

I have the following lines:

selenium.click("id=RadioButton")
selenium.waitForPageToLoad('30000')
selenium.type("id=Textbox", "patient is in a lot of pain")
assertTrue(selenium.isEditable("id=Textbox"));

The system should click the radiobutton, type some text in the texbox, and finally perform an assertion that the textbox is editable. It performs the first two just fine, but then fails the assertion! How is that possible?

Thanks.