Unexpected Error on ItemSearch

When I attempt to do an ItemSearch on a range of items that are part of a parent-child relationship, I receive an Unexpected Error. For example, the code below will return an Unexpected Error:

ItemSearch itemSearch = new ItemSearch();
itemSearch.setBasic(new ItemSearchBasic());
 
itemSearch.getBasic().setName(new SearchStringField("1",SearchStringFieldOperator.startsWith));
 
SearchResult result = null;
try {
result = _port.search(itemSearch);
} catch {
      ...


However if I change the search constraint from "starting with 1" to "starting with 5" the records are returned properly as one would expect. The difference being my items starting with "1" are grouped using a parent-child relationship and my items starting with "5" have no such relationship. If I further narrow down the search criteria to include just the child items of a parent-child relationship, the search will return results correctly as well. It is only when a parent item is included in the search results that I receive the Unexpected Error.