The objects with a custom shader become invisible when i activate the lightmap

this also happens when I use a very simple shader like this:

  Shader "Custom/shader00"

      {

          SubShader

          {

              Pass

              {

                  Material

                  {

                      Diffuse (3,3,3,1)

                      Ambient (3,3,3,1)

                  }

                  Lighting On        

              }

          }

      }