INSERT with SELECT not working in pgAdmin

Hi guys,

I'm trying to merge two tables to one with INSERT and SELECT commands:

INSERT into c_transactions (timestamp) SELECT entrytimestamp from c_transactions

INSERT into c_transactions (timestamp) SELECT exittimestamp from c_transactions

But getting this error on execution (in pgAdmin): "ERROR: null value in column "id" violates not-null constraint
SQL Status:23502"

What I'm doing wrong or is there another possibility for table merging?

I'm using postgres 8.3.

Thanks.