Error occurs when try to use Protege4.0.2

Hello,
I am getting the following error message when I try to use Protege4.0.2 following the MachesterTutorial and when I try to classify the ontology. The error shows “InconsistentOntologyException: Inconsistent ontology”.
How can I avoid this annoying error message?
Any idea would be helpful.
Thanks.

Reasoner Error
InconsistentOntologyException: Inconsistent ontology
