map export resolution select may be causing error

[jywarren]

I see this in the log:

ActionView::TemplateError (undefined method `/' for nil:NilClass) on line #10 of app/views
/map/_export_options.html.erb:
7: end
8: hist = hist[0..100]
9: grouphist = @map.grouped_images_histogram((hist.length/15).to_i+1)
10: resolution = @map.average_cm_per_pixel.to_s[0..4]
11: %>
12: 
13: <p><small>Slide to select an export resolution:</small></p>

[jywarren]

haven't seen this recently.