RaycastHit2D.collider is null - why?

I've already looked at the docs, which say that 'collider' is only null when no collider is hit -- let me show you why I think that's impossible.

Here's the code (C#):

 public Transform closestSeenHuman() {
         float distanceToClosestSeen = visionRange;
         
         Transform closestSeen = null;
         
         foreach (Transform t in info.targets) {
             float dist = Vector2.Distance((Vector2)transform.position, (Vector2)t.position);
             if (dist < distanceToClosestSeen) {
                 float angle = Vector2.Angle (Utils.forward2D(transform, 'd'), t.position - transform.position);
                 if (angle < fovAngle * 0.5f) {

                                     //Here's the part that matters:
                     Vector2 direction = (Vector2) t.position - (Vector2) eyes.position;
                     RaycastHit2D hit = Physics2D.Raycast((Vector2)eyes.position, direction, Mathf.Infinity, ignoreEnemy.value);

                                     //this prints every frame that the human is in view
                     if (hit.collider == null) print("null collider");
                     
                     Debug.DrawLine(eyes.position, t.position);
                     
                     if (hit.collider.transform == t) {
                         distanceToClosestSeen = dist;
                         closestSeen = t;
                         print("seen");
                     }
                 }
             }
         }
         
         return closestSeen;
     }

The idea of the function is that if the human is within the scriptholder's vision parameters (both angle and distance) then a raycast is performed which, mathematically speaking, should hit the human or at least something in front of the human!

Here's my scene view: (turns out UA doesn't take images, so I posted a screenshot on Imgur) http://i.imgur.com/CyYAWdY.png

Visually speaking, the raycast has hit the human, and the human has a circle collider centered on its pivot point. What's going on?

Bonus points go to the brave soul who can explain to me why the Zombie's arms get in the way of its sight, even with the layermask.