5000: Error migrating from CS3 to CS4

I have moved more than 1 Gaia Framework file from Flash CS3 to CS4 and got this error.

5000: The class ‘Work’ must subclass ‘flash.display.MovieClip’ since it is linked to a library symbol of that type.

The solution is to import the MovieClip class into the import area and to extend the main class as a MovieClip.

In my case from this:

public class Logger
{

to this:

public class Logger extends MovieClip
{