spsite data query sorting

Hello everyone; 

I want to get last 3 posts of a blog subsite and when I sort by PublishedDate my query fails.

ex: 

 query.Query = "<OrderBy>" +
                           "<FieldRef Name=\"PublishedDate\" Ascending=\"FALSE\"/>" +
                       "</OrderBy>";

this one gives me native stack error but, If i order by with ID or Title it works. 

any solutions for this?