Custom properties not working when extending ContentBySearchWebPart?

I'm extending ContentBySearchWebPart to have a few custom properties needed for the display templates. However, when creating a webpart and extending ContentBySearchWebPart, the custom properties doesn't show up when editing the webpart.

Properties doesn't show up:

public class ContentBySearchWithLink : ContentBySearchWebPart
{
	[Personalizable(), 
	WebBrowsable, 
	Category("Our Custom Properties"), 
	WebDisplayName("CustomLinkUrl")]
	public string CustomLinkUrl { get; set; }
}

Properties show up:

public class ContentBySearchWithLink : Microsoft.SharePoint.WebPartPages.WebPart
{
	[Personalizable(), 
	WebBrowsable, 
	Category("Our Custom Properties"), 
	WebDisplayName("CustomLinkUrl")]
	public string CustomLinkUrl { get; set; }
}

Any ideas??

EDIT: According to http://msdn.microsoft.com/en-us/library/microsoft.office.server.search.webcontrols.contentbysearchwebpart.aspx the ContentBySearchWebPart is a descendant of Microsoft.SharePoint.WebPartPages.WebPart which makes it a bit more strange.