Testfor dosen't work in a square unless r=2 or more.

I am trying to testfor a player within a square field.
I use /testfor @p

{100,56,100,110,58,110,r=0}

but It does nothing. I also try r=1 but it also does nothing. r=2 does do it, but it uses a bigger area than specified. I don't know if it is a bug or if I did something wrong and I have already tried x=, y=, z=, dx=, dy=, dz but no luck. Please fix as I need to use this feature in a map.