Hello, I'm trying to do the Using Simple Variables and Formulas challenge from Intro to Programmatic App Development. Is asking for:
Create a Visualforce page that displays the first name of the logged-in user.
I created a VF page named as is requested, with the following code:
<apex:page>
    {! $User.FirstName } 
</apex:page>
But when trying to check the challenge, I get the error:
Challenge not yet complete... here's what's wrong: 
The Visualforce page did not fit the criteria. The first name of the logged in user was not output using dynamic global variables.

What am I doing wrong?
Thank you!