To reproduce:
1) Create new project
2) Add a plane
3) Bake a NavMesh on it through Navigation panel
4) Attach a script with this in either Start() or Update() to a GameObject (Main Camera works):
//C#
NavMeshHit hit;
NavMesh.SamplePosition(transform.position, out hit, float.PositiveInfinity, 1);
Debug.Log("Hit: " + hit.hit);
Debug.Log("Position: " + hit.position);
Debug.Log("Distance: " + hit.distance);

5) Play the scene, notice that there was no hit.
If you change float.PositiveInfinity to a number, it gets a hit.