Trying to create a sub category using java api

            parentCategoryHandle = service.findCategoryByNamePath(project,namePath, MONITOR);
            if (parentCategoryHandle == null) {
                return "ERROR: Category " + categoryName + " parent not found";
            }
            // Create category
  (error line 272)          category = service.createSubcategory(parentCategoryHandle,categoryName,MONITOR);

Error I get is:
Exception in thread "main" org.eclipse.core.runtime.AssertionFailedException: assertion failed:
    at org.eclipse.core.runtime.Assert.isTrue(Assert.java:110)
    at org.eclipse.core.runtime.Assert.isTrue(Assert.java:96)
    at com.ibm.team.workitem.common.internal.util.CategoriesHelper.createCategoryId(CategoriesHelper.java:175)
    at com.ibm.team.workitem.common.internal.WorkItemCommon.createSubcategory(WorkItemCommon.java:738)
    at interations_releases.Repository.createCatagory(Repository.java:272)