basic temp table question

Hello,

The first time I run a query against this function (using Npgsql):

declare
r_cursor1 cursor for

SELECT * from tmp_table;

begin

CREATE TEMPORARY TABLE tmp_table
(
testcol integer
) ON COMMIT DROP;

INSERT INTO tmp_table
SELECT 0;

open r_cursor1;

return 0;
end;

I get this error:

ERROR: XX000: relation 1090457025 is still open

All subsequent times I get this error:

ERROR: 42P01: relation with OID 1090457025 does not exist

What am I doing wrong?

(Sorry if this is a basic question.)

Thanks in advance...