I encountered this error when I set up my fresh SharePoint 2010 farm and created the site collection before completing the configuration of the search service.
I checked all my settings, but the search service was up and running and presented no errors. Still, I was not able to fire a query from code, as this exception kept popping up. After doing a search on internet, I found that the problem was not with the search service, but with the content database settings. Strange place I would say. It appears though that each content database is assigned its own indexer and because the search service was turned on after the content database had been created, no indexer was assigned to the content database.
So, to correct this problem, open Central Admin, go to Manage Content Databases, select your content database and select the appropiate indexer in the properties page. Once done, the problem was gone.
Easy enough right?