Index: lucene/src/java/org/apache/lucene/util/FixedBitSet.java
===================================================================
--- lucene/src/java/org/apache/lucene/util/FixedBitSet.java	(revision 1174017)
+++ lucene/src/java/org/apache/lucene/util/FixedBitSet.java	(working copy)
@@ -139,9 +139,12 @@
    *  -1 is returned if there are no more set bits.
    */
   public int nextSetBit(int index) {
-    assert index >= 0 && index < numBits;
+    // we allow length() as index to support the following for-loop:
+    //  for(int i=bs.nextSetBit(0); i>=0; i=bs.nextSetBit(i+1)) {}
+    assert index >= 0 && index <= numBits: "index=" + index + " numBits=" + numBits;
     int i = index >> 6;
-    int subIndex = index & 0x3f;      // index within the word
+    if (i >= bits.length) return -1;
+    final int subIndex = index & 0x3f;      // index within the word
     long word = bits[i] >> subIndex;  // skip all the bits to the right of index
 
     if (word!=0) {
@@ -158,13 +161,17 @@
     return -1;
   }
 
+  /** Returns the index of the last set bit before or on the index specified.
+   *  -1 is returned if there are no more set bits.
+   */
   public int prevSetBit(int index) {
-    assert index >= 0 && index < numBits: "index=" + index + " numBits=" + numBits;
+    // we allow -1 as index to support the following for-loop:
+    //  for(int i=bs.prevSetBit(bs.length()-1); i>=0; i=bs.prevSetBit(i-1)) {}
+    assert index >= -1 && index < numBits: "index=" + index + " numBits=" + numBits;
     int i = index >> 6;
-    final int subIndex;
-    long word;
-    subIndex = index & 0x3f;  // index within the word
-    word = (bits[i] << (63-subIndex));  // skip all the bits to the left of index
+    if (i < 0) return -1;
+    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) {
       return (i << 6) + subIndex - Long.numberOfLeadingZeros(word); // See LUCENE-3197
Index: lucene/src/test/org/apache/lucene/util/TestFixedBitSet.java
===================================================================
--- lucene/src/test/org/apache/lucene/util/TestFixedBitSet.java	(revision 1174017)
+++ lucene/src/test/org/apache/lucene/util/TestFixedBitSet.java	(working copy)
@@ -37,7 +37,7 @@
     int aa=-1,bb=-1;
     do {
       aa = a.nextSetBit(aa+1);
-      bb = bb < b.length()-1 ? b.nextSetBit(bb+1) : -1;
+      bb = b.nextSetBit(bb+1);
       assertEquals(aa,bb);
     } while (aa>=0);
   }
@@ -58,7 +58,7 @@
       } else if (bb < 1) {
         bb = -1;
       } else {
-        bb = bb >= 1 ? b.prevSetBit(bb-1) : -1;
+        bb = b.prevSetBit(bb-1);
       }
       assertEquals(aa,bb);
     } while (aa>=0);