One of the Java 5 method overrides wasn't commented out, and its return value conflicts with Appendable. This could cause problems.
diff --git a/src/main/java/freemarker/template/utility/NullWriter.java b/src/main/java/freemarker/template/utility/NullWriter.java
index cf0ff2a..6587c31 100644
--- a/src/main/java/freemarker/template/utility/NullWriter.java
+++ b/src/main/java/freemarker/template/utility/NullWriter.java
@@ -54,11 +54,11 @@
         // Do nothing
         return this;
     }
-    */
 
     public Writer append(char c) throws IOException {
         // Do nothing
         return this;
     }
+    */
 
 }