Appeal assessment for late NR6 filing (under section 216 - non-resident property owner)

I was under the misapprehension that I had two years to file tax returns under section 216. I now realize that because of my NR6 submission I had to file by June 30th.

I was notified this week that CRA is assessing me $6K (taxing my gross income, instead of net) because I filed in September instead of June.

I would like to appeal, but was told by a CRA agent that misunderstanding is not reason enough for relief or forgiveness... any advice as to whether any consideration might be given in my case, or what circumstances would need to exist for consideration to be given?