Index: queryparser/src/java/org/apache/lucene/queryparser/flexible/core/builders/QueryTreeBuilder.java
===================================================================
--- queryparser/src/java/org/apache/lucene/queryparser/flexible/core/builders/QueryTreeBuilder.java	(revision 1486282)
+++ queryparser/src/java/org/apache/lucene/queryparser/flexible/core/builders/QueryTreeBuilder.java	(working copy)
@@ -150,7 +150,7 @@
         builder = getQueryBuilder(clazz);
 
         if (builder == null) {
-          Class<?>[] classes = node.getClass().getInterfaces();
+          Class<?>[] classes = clazz.getInterfaces();
 
           for (Class<?> actualClass : classes) {
             builder = getQueryBuilder(actualClass);
Index: queryparser/src/test/org/apache/lucene/queryparser/flexible/core/builders/TestQueryTreeBuilder.java
===================================================================
--- queryparser/src/test/org/apache/lucene/queryparser/flexible/core/builders/TestQueryTreeBuilder.java	(revision 1486282)
+++ queryparser/src/test/org/apache/lucene/queryparser/flexible/core/builders/TestQueryTreeBuilder.java	(working copy)
@@ -22,6 +22,8 @@
 import org.apache.lucene.queryparser.flexible.core.QueryNodeException;
 import org.apache.lucene.queryparser.flexible.core.nodes.FieldQueryNode;
 import org.apache.lucene.queryparser.flexible.core.nodes.QueryNode;
+import org.apache.lucene.queryparser.flexible.core.nodes.QueryNodeImpl;
+import org.apache.lucene.queryparser.flexible.core.parser.EscapeQuerySyntax;
 import org.apache.lucene.queryparser.flexible.core.util.UnescapedCharSequence;
 import org.apache.lucene.util.LuceneTestCase;
 import org.junit.Test;
@@ -35,8 +37,29 @@
     Object result = qtb.build(new FieldQueryNode(new UnescapedCharSequence("field"), "foo", 0, 0));
     Assert.assertEquals("OK", result);
     
+    qtb = new QueryTreeBuilder();
+    qtb.setBuilder(DummyQueryNodeInterface.class, new DummyBuilder());
+    result = qtb.build(new DummyQueryNode());
+    Assert.assertEquals("OK", result);
   }
   
+  private static interface DummyQueryNodeInterface extends QueryNode {
+    
+  }
+  
+  private static abstract class AbstractDummyQueryNode extends QueryNodeImpl implements DummyQueryNodeInterface {
+    
+  }
+  
+  private static class DummyQueryNode extends AbstractDummyQueryNode {
+
+    @Override
+    public CharSequence toQueryString(EscapeQuerySyntax escapeSyntaxParser) {
+      return "DummyQueryNode";
+    }
+    
+  }
+  
   private static class DummyBuilder implements QueryBuilder {
 
     @Override