Blob not found problem

I have a program using jdbc and informix 2000 9.2x. I select a blob from informix database. I got the error Blob not found. But strange thing, one of my thread has got this blob and in the thread getting error message, it can get a value of a column which is in the same record as the blob value I need to get. for example in a record, it contains two value, one is name, another is blob. the thread can get name value, but when it try to get blob value, it fails. It is possible that the record has been delete. but the problem is I use serializable transaction(ansi equal to repeatable read isolation level). So, I think this thing should not happen. Is it a informix bug? Does anybody like to tell me the reason? Thanks in adavance.