A minor change of fNewLine.toString() which
was wrong, to String.valueOf(fNewLine)); to 
turn the character array into a String.

Thanks to Neil Delima for pointing out this error.
diff --git a/src/org/apache/xml/serializer/dom3/DOM3TreeWalker.java b/src/org/apache/xml/serializer/dom3/DOM3TreeWalker.java
index 59dcae7..9b5af71 100644
--- a/src/org/apache/xml/serializer/dom3/DOM3TreeWalker.java
+++ b/src/org/apache/xml/serializer/dom3/DOM3TreeWalker.java
@@ -2140,7 +2140,7 @@
         // Set the newLine character to use

         if (fNewLine != null) {

             fSerializer.setOutputProperty(OutputPropertiesFactory.S_KEY_LINE_SEPARATOR,

-                    fNewLine.toString());

+                    String.valueOf(fNewLine));

         }

     }