Adding a property on http://schema.org/Offer

• From: <gogos0@freemail.gr>
• Date: Mon, 5 Nov 2012 11:13:58 +0200
• To: public-vocabs@w3.org
• Message-Id: <50978356767fa2.70815057@freemail.gr>

Hi, I'm new on this subject and not sure if I'm doing something wrong:

I need to use an extra property "category" on the existing http://schema.org/Offer.

I added then line:<meta itemprop="category" content="Photography">

When  testing with http://www.google.com/webmasters/tools/richsnippets the new property is read but I get the error:
Warning: Page contains property "category" which is not part of the schema.


Is this ok?

Thank you.

Full code:

<tr itemscope="" itemtype="http://schema.org/Offer">                     
                     <meta itemprop="category" content="Photography">
                                        
                    <span itemprop="price">R89</span>
                    <span itemprop="description">Whimsical, Hand Printed Tea Towels</span></span> - Offer expires: <b>09/11/2012 06:01</b> <meta itemprop="priceValidUntil" content="2012-11-09T06:01:00">
                                    </tr> 

Received on Monday, 5 November 2012 21:43:20 UTC