Tilestache: Not able to render data from postGIS SQL

Question

my tilestache.cfg is as below:

{
  "cache": {
        "name": "Disk",
        "path": "mnt/tile-cache/"
    }
    ,    
  "layers":
  {  
     "countries":
    {
      "provider": {
            "name": "vector", 
            "driver": "PostgreSQL",
            "parameters": 
             {
            "dbname": "test1",
            "host" : "localhost",
            "port" : "5432",
            "user": "postgres",
                        "password":"postgres",
                        "table": "ne_110m_admin_0_countries",
                         "query":"SELECT admin, ST_Y(ST_Centroid(wkb_geometry)) as lattitude FROM ne_110m_admin_0_countries;"

             }}}}}

I am getting error as

File "/usr/local/lib/python2.7/dist-packages/TileStache-1.51.6-py2.7.egg/TileStache/__init__.py", line 303, in requestHandler2
    print >> out, 'Known unknown!'

---

I am using URL as localhost:8080/countries/1/1/1.json

please help me with rendering tile.