Have you tried to go with the “Safe usage” alternative given in the commented section?
I honestly don’t have any experience with wp_get_current_user(), since I never use it, but anyhow, this ought to work:
global $current_user;
echo 'Username: ' . $current_user->user_login . '<br />';
echo 'User email: ' . $current_user->user_email . '<br />';
echo 'User first name: ' . $current_user->user_firstname . '<br />';
echo 'User last name: ' . $current_user->user_lastname . '<br />';
echo 'User display name: ' . $current_user->display_name . '<br />';
echo 'User ID: ' . $current_user->ID;
wp_get_current_user() should do the same, as it is nothing but a wrapper for the first two lines above, nonetheless, the above has got to work.