MySQL: Invalid use of group function

 I am using MySQL. Here is my schema:

Suppliers(sid: integer, sname: string, address string)

Parts(pid: integer, pname: string, color: string)

Catalog(sid: integer, pid: integer, cost: real)

(primary keys are bolded)

I am trying to write a query to select all parts that are made by at least two suppliers:

-- Find the pids of parts supplied by at least two different suppliers.
SELECT c1.pid                      -- select the pid
FROM Catalog AS c1                 -- from the Catalog table
WHERE c1.pid IN (                  -- where that pid is in the set:
    SELECT c2.pid                  -- of pids
    FROM Catalog AS c2             -- from catalog
    WHERE c2.pid = c1.pid AND COUNT(c2.sid) >= 2 -- where there are at least two corresponding sids
);

First off, am I even going about this the right way?

Secondly, I get this error:

1111 - Invalid use of group function

What am I doing wrong? Why getting error mysql invalid use of group function?