Question How do I get rid of these awful depth-of-field artifacts?

You guessed it. I need help, please.
I've got a camera, using depth of field, looking past a cube that's really close to it (and hence very blurry) at a wall that's further away.
As you can see, I'm not getting a nicely blurred transition from the cube to the wall as one would expect.
Something seems very wrong with the camera's depth texture (at least that's my guess), and there's a really sharp dilated outline visible around the blurred object. (It's not following the exact outline of the geometry. As I said, it's slightly dilated.)

Can this be fixed at all?
Does it qualify as a bug? Are there any Unity developers present?