Topic: implementing a threaded forum

Am trying to implement a threaded forum using the acts_as_threaded plugin. However i get  this error:

RuntimeError: ERROR    C55000    
Mcurrval of sequence "forumpost_forumpost_id_seq" is not yet defined in this session   
Fsequence.c    L626    Rcurrval_oid: SELECT currval('forumpost_forumpost_id_seq')

am using postgres 8.1 server

what could be the source of this error?

thank you