How to get text out of div class tag in selenium webdriver?

<div class="error-text">invalid login or password.</div>

HOw do I retrieve the actual text "invalid login or password"? I m running the test on testNG. Here is my test java code:

WebElement wrong_message = firefox.findElement(By.xpath("//*[@id=\"login-form\"]/div[3]"));
      System.out.println("message is here:" + wrong_message.getText().toString());
      Assert.assertNotNull(wrong_message);

But the output of testrun has empty string instead of "invalid login or password".