Empty result of query about gitbase HOT 4 CLOSED

SELECT
    files.repository_id,
    files.file_path,
    UAST(files.blob_content, LANGUAGE(files.file_path, files.blob_content), '//uast:FunctionGroup') as functions
FROM files
NATURAL JOIN commit_files
NATURAL JOIN commits
NATURAL JOIN refs
WHERE
    refs.ref_name= 'HEAD'
    AND functions IS NOT NULL
    AND LANGUAGE(files.file_path,files.blob_content) = 'Java'
LIMIT 10;