Adjacent vertices always zero in Geometry shader

Question

Just a simple geometry shader that outputs its input, to test the hlsl syntax.

struct GSOutput
{
    float4 pos : SV_POSITION;
};
[maxvertexcount(2)]
void main(
    lineadj float4 adj[4] : SV_POSITION,
    inout LineStream<GSOutput> output
)
{
    GSOutput element;
    element.pos = adj[0];
    output.Append(element);
    element.pos = adj[1];
    output.Append(element);
}

The docs say that v[1], v[2] should be the vertices of the line, but I'm getting v[0],[1] as valid vertices of the line and v[2], v[3] always zero? Did I do something wrong in the shader.

This is instanced and indexed draw call. Line strip topology.

What happens when there is no adjacency? Can we detect it? I'm trying to compute a moving median on a line strip.