<my:RadTabItem.Content> not a recognised tag

[G S S]

Hi,

I am using the latest version of silverlight controls in VS2010 Ultimate.

When i use the tabcontrol, with the below tag, it is not found (the Content part), and thus breaks the design:

            <my:RadTabItem.Content>

Error in the designer:

The document contains errors that must be fixed before the designer can be loaded. Reload the designer after you have fixed the errors.

Any idea why this problem occurs?

Thanks