Is there a limit to iterating over arrays in Jade/Express?

I'm trying to iterate over an nest of arrays in Jade. I seem to have hit a limit.

div().slidePreview
  ul
    each slide in slides
      div
        each section in slide
          li
            each image in section
              img(class= image.orientation, src='http://stevepainter.s3.amazonaws.com/images/thumbs/' + image.filename)

One less iteration and it works fine (i.e. each section in slides[0]). If I console.log(image) I get the full object. If I console.log(image.orientation) I get the orientation but in the browser I get:-

18| each image in section 19| -console.log(image.orientation) 20| img(class= image.orientation, src='http://stevepainter.s3.amazonaws.com/images/thumbs/' + image.filename) 21| Cannot read property 'orientation' of undefined> 18| each image in section 19| -console.log(image.orientation) 20| img(class= image.orientation, src='http://xxxxxxxxxxxxx.s3.amazonaws.com/images/thumbs/' + image.filename) 21| Cannot read property 'orientation' of undefined>

Please help this is killing me!!!!