(require) is still required, even after (package-initialize)

[Pankaj Jangid]

The documentation says that (require 'package) is not required if
(package-initialize) is already invoked in init.el.

But flymake is reporting "server-running-p is not known to be defined"
in the below statement,

(unless (server-running-p) (server-start))

I have also tried to call,

(eval-when-compile (package-initialize)) instead of just
(package-initialize). But still flymake is reporting the error.

(require 'server) solves the error.

Am I using package-initialize wrongly?

-- 
Pankaj Jangid