ERROR: heap_fetch: xinv19073 relation: ReadBuffer(81aeefe) failed

Hello PostgreSQL Hackers,

I get this error with the attached program. Is this a bug or is it that
I'm doing something totally stupid?

The program opens a connection, starts a transaction, creates a large
object with mode INV_READ | INV_WRITE, opens it with the same mode, calls
lo_write five times with a single 'X', calls lo_lseek to seek to position
1, calls lo_write to write a single 'y', then calls lo_lseek to seek to
position 3 and writes another 'y', then calls lo_close and finally ends
the transaction. The error happens when I write the second 'y'.

I have attached a trace to this message too.

Thanks in advance for your time.

Ian