sql*plus shows null on pk cols during desc

I vaguely remember coming across this in the past but I can't find anything in metalink or google so I thought I would ask here.  When I do a desc of a table in sql*plus it shows pk columns as being null when they should be not null.  However, when I check dba_constraints I see the pk constraint on the table and it's enabled.  So is this a sql*plus bug??

desc FORMAT_SCHEMA.FACSIMILE_LEVEL2_RECORD

Name                                      Null?    Type

----------------------------------------- -------- ----------------------------

FACSIMILE_HEADER_ID                                NUMBER(10)

FAX_LEVEL1_REC_SQNC_NBR                            NUMBER(4)

FAX_LEVEL2_REC_SQNC_NBR                            NUMBER(4)