Why does getting the IAttribute value throw AssertionFailedException ?

Hello All,

I have following code, which throws AssertionFailedException. What is the reason for this.

workItemClient = (IWorkItemClient) teamRepository .getClientLibrary(IWorkItemClient.class);
        IEnumeration<ILiteral> enumeration = (IEnumeration<ILiteral>) workItemClient.resolveEnumeration(iattribute, monitor);
            iac = (IAuditableClient) ((ITeamRepository) currentWorkItem.getOrigin()).getClientLibrary(IAuditableClient.class);
            String[] iaval = iattribute.getValue(iac, currentWorkItem, monitor);

Request you to assist me on this.

Thanks