The mobs attack players through the corner

 I stand in a corner of a cave against two walls, and I stack two blocks to my left and right. The zombie's hand clips through the missing corner of the baricade and punches me, but I can't hit it! This is a drawing of what I am trying to explain.

 

*Edit

So, let me explain this since the picture can't be viewed. Sorry about that. So, I place down blocks to the left and right of me, and I don't fill in the corner. As a result, if I am in a one block space, the mob's hand clips through the empty corner and deals damage. No matter how far back or forward I move in the one block space, the mob hits me, and I die. This is extremely inconvenient for when I have no pickaxe or wood. I don't really have any pictures other than something I could draw, sorry.