Javadoc generation failed with vaadin dependency

I have a small project that has vaadin as dependency. The build works fine. Other projects without vaadin as dependeny can create the javadocs.

When i want to create the javadocs task, the following error is given:

src\components\bpmn-utils>gradle javadoc

Building > :bpmn-utils:compileJava > Resolving dependencies ':bpmn-utils:compi
:bpmn-utils:compileJava UP-TO-DATE
:bpmn-utils:processResources UP-TO-DATE
:bpmn-utils:classes UP-TO-DATE
:bpmn-utils:javadoc
C:\Documents and Settings\name.gradle\caches\artifacts-13\filestore
com.vaadin\vaadin.8.0\jar\a8565de65480c3fde69468774792f97d4214c678\vaadin-6.8.
0.jar(com/vaadin/ui/ClientWidget.java):26: error: cannot access Paintable
import com.vaadin.terminal.gwt.client.Paintable;
^
1 error
bad source file: C:\Documents and Settings\name.gradle\caches\arti
facts-13\filestore\com.vaadin\vaadin.8.0\jar\a8565de65480c3fde69468774792f97d4
214c678\vaadin-6.8.0.jar(com/vaadin/terminal/gwt/client/Paintable.java)
file does not contain class com.vaadin.terminal.gwt.client.Paintable
Please remove or make sure it appears in the correct subdirectory of the sou
rcepath.
FAILURE: Build failed with an exception.

  • What went wrong:
    Execution failed for task ‘:bpmn-utils:javadoc’.

Javadoc generation failed.
Here the dependencis

dependencies {
// Activiti
compile group:‘org.activiti’, name:‘activiti-engine’, version:‘5.9’
// Vaadin
compile group:‘com.vaadin’, name:‘vaadin’, version:‘6.8.0’
}
Maybe this is a bug in vaadin? Or can i disbable the dependencies for the javadoc task?

Thanks