resource busy error (00054) when doing an 'alter table... add parition'

I am getting a resource busy error (00054) when doing an 'alter table... add parition'. I am adding a new parition to the table, I don't understand why this will be not permitted, even if someone else is manipalating data in the other partitions ?

SQLWKS> alter table tad.quote add partition tad_quote_data_w20001204
2> values less than ('20001211') tablespace tad_quote_data_05;
alter table tad.quote add partition tad_quote_data_w20001204
*
ORA-00054: resource busy and acquire with NOWAIT specified