Sorting with Zend Lucene

I'm having a rather drastic ussing with sorting using Zend Search Lucene.  Our index has 18,000 documents, so light as far as Lucene is concerned.  I can perform a search that will pull back all of our records with no problem.

 

However, once I attempt to add a sort field (Sorting by Title for instance), the script crashes with an out of memory exception.  What is the best way to handle sorting to avoid running out of memory?