Generate Documentation of DTD

I have a DTD and .ent files, want to generate documentation of html format, for this i tried using DTDDoc.
didn't generated throwing error for my DTD and .ent files, where as able to generate for sample.
could you suggest what could cause this error, is there any better tool to generate.

Errror is

C:\DTDDoc_1_1_0\DTDDoc>ant
Buildfile: build.xml

compile:

buildExample:

main:
   [DTDDoc] Welcome to DTDDoc 1.1.0 (2009-11-02).
   [DTDDoc] Parts of this program are from DTDParser and RegExp of the Apache Pr
oject.
   [DTDDoc] The settings are :
   [DTDDoc]    Don't show @hidden tags.
   [DTDDoc]    Don't show @fixme tags.
   [DTDDoc]    No style sheet was specified.
   [DTDDoc]    Source directory is 'C:\DTDDoc_1_1_0\DTDDoc\example'.
   [DTDDoc]    Destination directory is 'C:\DTDDoc_1_1_0\DTDDoc\example_doc'.
   [DTDDoc] Parsing 'C:\DTDDoc_1_1_0\DTDDoc\example\MedInfo_v1.0\MedInfo_v1.0.dtd'...
   [DTDDoc] WARNING: locateElements():can't read C:\DTDDoc_1_1_0\DTDDoc\example\MedInfo_v1.0\MedInfo_v1.1.ent
   [DTDDoc] ERROR: A problem occured while instanciating or executing the DTDDoc
 task !
   [DTDDoc] ERROR: java.lang.NullPointerException
     [echo] Now you can browse the result from "example_doc/index.html".

main:

BUILD SUCCESSFUL
Total time: 1 second