Failing to create relationship (SQL Server claims table lacks PK for some reason ?)

[pj]

I tried to create a relationship in EM's diagram pane in
SQL Server 2000 (I'd list the version of EM, but About gives me
the MMC version, which is probably not relevant.)

(The database itself is SQL Server 2000 SP3.)

I got an error that I don't understand (because, at least
at first blush, it appears to be quite untrue). Note that
I have never clicked before on the diagrams child of the
database; this was entirely experimental.

"Primary key or UNIQUE constraint must be defined for
table 'xxx' before it can participate in a relationship."

http://msdn.microsoft.com/library/de...antbepktbl.asp
1) My table already had a primary key; why is MS SQL Server apparently
claiming otherwise ? (I don't think I can define a second primary key
on the same table. I could perhaps define an additional unique index
on top of the primary key, but, I'm not sure.)

2) What does it mean: table '<0s>' ? That is, what does 0s mean ?