Nikon DF: images shot on an FX camera with a DX lens can't be edited

Nikon cameras, when using a DX (crop sensor) lens on an FX (full frame)
camera DT is unable to load images into darkroom module.

To Reproduce

1. import images shot with a Nikon FX camera with DX lens
2. Click on 'Darkroom'

Expected behavior
image opens for editing

Screenshots
DT_error.jpg

Platform
_Please fill as much information as possible in the list given below. Please state "unknown" where you do not know the answer and remove any sections that are not applicable _

• darktable version : 3.4.1
• OS : Linux - kernel 5.12.1
• Linux - Distro : Arcolinux rolling
• Memory : 64gb
• Graphics card : AMD Radeon RX480
• Graphics driver :
• OpenCL installed :
• OpenCL activated :
• Xorg : X11
• Desktop : i3wm
• GTK+ :
• gcc :
• cflags :
• CMAKE_BUILD_TYPE :

Additional context
Please provide any additional information you think may be useful, for example:

• Can you reproduce with another darktable version(s)? yes with version x-y-z / no
• Can you reproduce with a RAW or Jpeg or both? RAW-file-format/Jpeg/both
• Are the steps above reproducible with a fresh edit (i.e. after discarding history)? yes/no
• If the issue is with the output image, attach an XMP file if (you'll have to change the extension to .txt)
• Is the issue still present using an empty/new config-dir (e.g. start darktable with --configdir "/tmp")? yes/no