texture coordinate generation

I have problems with texture coordinate generation in a vertex shader (using GL_EXT_vertex_shader).
What I try to do is this:
glShaderOp2EXT(GL_OP_DOT3_EXT, GL_OUTPUT_TEXTURE_COORD#_EXT, lightDir, eyeNormal);
This always produces zero as output coordinate when # > 0. But if I send the same dot product as output color, I get the expected value.