Need help about rendering depth map using Mitsuba

Hi, there,

I just got a problem about generating the depth map using Mitsuba. In the xml file, I did like this:

<integrator type="multichannel">
  <integrator type="field">
      <string name="field" value="distance"/>

But the rendering view is like black and white. I output it as exr and read in Matlab it is Nan in black area and Inf in white.

Any help will be appreciated,