Usage of non-serializable class in custom server side component

Hi, Vaadin people,

I found that using static non-serializable classes in server side components like this:

import org.apache.log4j.Logger;
private static final Logger log = Logger.getLogger(FileUploadBase.class);

causes the following error on the client side:

Widgetset does not contain implementation for com.netshop.ui.widget.upload.FileUploadBase. Check its @ClientWidget mapping, widgetsets GWT module description file and re-compile your widgetset. In case you have downloaded a vaadin add-on package, you might want to refer to add-on instructions. Unrendered UIDL:
com.netshop.ui.widget.upload.FileUploadBase(NO CLIENT IMPLEMENTATION FOUND)

I wonder why static variable of the server side component affects client side?
Also I do not get any widget compilation errors.

I feel that missing something important.

Any help would be appreciated.