ECPG ERROR FOR "exec sql BEGIN"

I have used ECPG in my program.

but run "exec sql BEGIN" after connect, have a error message like:

"sql error BEGIN: already a transaction in progress"

the source is like:

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exec sql connect to unix:postgresql://localhost/dbname user postgres;
exec sql BEGIN;

exec sql INSERT INTO .......

exec sql COMMIT;
exec sql disconnect;
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Help me. Thanks very much