SELECT FOR UPDATE NOWAIT

If I do :
Session 1:
decibel=# begin;
BEGIN
decibel=# select * from i where i=1 for update nowait;
i
---
1
(1 row)

decibel=# begin;
BEGIN
decibel=# select * from i where i=2 for update nowait;
i
---
2
(1 row)

Session 2 :

decibel=# select * from i where i=1 for update nowait;
ERROR: could not obtain lock on row in relation "i"
decibel=# select * from i where i=3 for update nowait;
ERROR: current transaction is aborted, commands ignored until end of
transaction block
decibel=# ==> Why ? :'(

Why i can't do any "Select for update" (current transaction aborted) after
having receive a not obtain lock ???
Thank you for your answer !

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