Mac clone manufacturer Psystar said that Apple's copyright suit against it should be dismissed because Apple has never filed for copyright protection for its Mac OS X operating system with the U.S. Copyright Office, according to court papers.

Apple "is prohibited from bringing action against Psystar for the alleged infringement of one or more of the plaintiff's copyrights for failure to register said copyrights with the copyright office as required" by law, Psystar claims.

The stunning claim, if true, could undermine Apple's ability to restrict third parties, such as Psystar, from selling clones that run the Mac OS on generic PC hardware. InformationWeek was not immediately able to verify the claim.

Psystar made the allegation in documents filed last week in U.S. District Court in San Francisco, as part of its response to Apple's latest charges of copyright infringement.

Psystar also claimed that Apple's Mac OS X 10.5 "Leopard" operating system contains undocumented code designed to render inoperable personal computers that aren't running on Apple-approved hardware.

Psystar claims Apple uses so-called stealthware to protect what Psystar claims is an illegal monopoly in the Mac computing market. Specifically, Psystar contends that OS X runs a startup routine that checks whether the host computer is running on a particular line of Intel dual-core processors that are included in genuine Macs.

Psystar sells unauthorized Mac clones from a nondescript warehouse in a Miami industrial park. Apple sued the company earlier this year for copyright violation. Psystar countersued in response, claiming that Apple's control of the Mac market violates antitrust laws.

Last month, a judge rejected Psystar's counterclaim -- leading Psystar to file revised claims. Psystar is now asking the judge overseeing the case to declare Apple's Mac OS copyrights invalid.

In court filings, Apple has said it believes Psystar is backed by a silent third party that's presumably seeking to enter the Mac market.