Index: src/java/org/apache/lucene/store/FSDirectory.java
===================================================================
--- src/java/org/apache/lucene/store/FSDirectory.java	(revision 312565)
+++ src/java/org/apache/lucene/store/FSDirectory.java	(working copy)
@@ -197,6 +197,7 @@
 
     String lockPrefix = getLockPrefix().toString(); // clear old locks
     files = lockDir.list();
+    if (files == null) throw new IOException("Cannot read lock directory "+lockDir.getAbsolutePath());
     for (int i = 0; i < files.length; i++) {
       if (!files[i].startsWith(lockPrefix))
         continue;