Index: /Users/jwang/projects/lucene_2_3_1/src/java/org/apache/lucene/search/BooleanQuery.java
===================================================================
--- /Users/jwang/projects/lucene_2_3_1/src/java/org/apache/lucene/search/BooleanQuery.java	(revision 641933)
+++ /Users/jwang/projects/lucene_2_3_1/src/java/org/apache/lucene/search/BooleanQuery.java	(working copy)
@@ -425,13 +425,18 @@
         buffer.append("+");
 
       Query subQuery = c.getQuery();
-      if (subQuery instanceof BooleanQuery) {	  // wrap sub-bools in parens
-        buffer.append("(");
-        buffer.append(c.getQuery().toString(field));
-        buffer.append(")");
-      } else
-        buffer.append(c.getQuery().toString(field));
-
+      if (subQuery!=null){
+	      if (subQuery instanceof BooleanQuery) {	  // wrap sub-bools in parens
+	        buffer.append("(");
+	        buffer.append(subQuery.toString(field));
+	        buffer.append(")");
+	      } else
+	        buffer.append(subQuery.toString(field));
+      }
+      else{
+    	  buffer.append("null");
+      }
+    	  
       if (i != clauses.size()-1)
         buffer.append(" ");
     }