Why is TreeWalk.forPath(...) returning null

My "git status" command shows:

# On branch master # Changed but not updated: # (use "git add <file>..." to update what will be committed) # # modified: xml/minDB/SubmitObjectsRequest_CMSScheme.xml

The file above was committed and then modified. I want to use jgit to find the comitted version of that file.

When I do the following code I get a null treeWalk? Why is that? What should I specify for path instead?

            String path = "xml/minDB/SubmitObjectsRequest_CMSScheme.xml";
            String versionName = Constants.HEAD;

            Commit commit = repository.mapCommit(versionName);

            if (commit == null) {
                log.trace("Did not find Commit. versionName:" + 
versionName);
            } else {
                ObjectId[] ids = {commit.getTree().getId()};
                TreeWalk treeWalk = TreeWalk.forPath(repository, path, ids);
            }

Thanks.