Index: lucene/src/java/org/apache/lucene/util/OpenBitSet.java
===================================================================
--- lucene/src/java/org/apache/lucene/util/OpenBitSet.java	(revision 1139353)
+++ lucene/src/java/org/apache/lucene/util/OpenBitSet.java	(working copy)
@@ -660,14 +660,16 @@
    *  -1 is returned if there are no more set bits.
    */
   public int prevSetBit(int index) {
-    if (index < 0) {
-      return -1;
-    }
-    int i = index>>6;
+    int i = index >> 6;
+    final int subIndex;
     if (i >= wlen) {
       i = wlen - 1;
+      if (i < 0) return -1;
+      subIndex = 0x3f; // last possible bit
+    } else {
+      if (i < 0) return -1;
+      subIndex = index & 0x3f;      // index within the word
     }
-    final int subIndex = index & 0x3f;      // index within the word
     long word = (bits[i] << (63-subIndex));  // skip all the bits to the left of index
 
     if (word != 0) {
Index: lucene/src/test/org/apache/lucene/util/TestOpenBitSet.java
===================================================================
--- lucene/src/test/org/apache/lucene/util/TestOpenBitSet.java	(revision 1139353)
+++ lucene/src/test/org/apache/lucene/util/TestOpenBitSet.java	(working copy)
@@ -42,8 +42,8 @@
   }
 
   void doPrevSetBit(BitSet a, OpenBitSet b) {
-    int aa=a.length();
-    int bb=aa;
+    int aa = a.size() + random.nextInt(100);
+    int bb = aa;
     do {
       // aa = a.prevSetBit(aa-1);
       aa--;