timestamp - timestamp causes null pointer exception

select tts.rnum, tts.cts - tts.cts from cert.tts tts

Error: Query returned non-zero code: 12, cause: FAILED: Hive Internal Error: java.lang.NullPointerException(null)
SQLState: 42000
ErrorCode: 12

create table if not exists CERT.TTS ( RNUM int , CTS timestamp)
stored as sequencefile;