Left join error

Hello.
I have a table with 146000+ records. I tried to select with a left join to
another table but I get this message:

ERROR: MemoryContextAlloc: invalid request size 4294967293

This is the query.

select * from cheques left join formasenblanco on cheques.cheque=
formasenblanco.cheque where formasenblanco.cheque is null;

The table cheques has about 146000+ records. Formasenblanco is smaller. I
know I can use subquerys, but in theory, this is much slower.
I'm using Postgres 7.1 on Redhat 6.2

Does anyone have a suggestion?

Thank you.

Ligia