[Freesurfer] Dividing surface overlays

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Hi all,

I've generated two surface overlays, using mri_vol2surf with positive and negative projfrac to get a GM and WM samples (gm_lh.nii.gz and wm_lh.nii.gz). I'd now like to take the vertex-wise ratio of these two overlays, and make a new one (gmwmratio_lh.nii.gz). I've tried doing this as:

fslmaths gm_lh.nii.gz -div wm_lh.nii.gz gmwmratio_lh.nii.gz

but the result is:

Image Exception: #22 :: Failed to read volume gm_lh.nii.gz Error : Error: Nonsensical ROI for gm_lh.nii.gz

Is there another tool I can use to compute this ratio? Is there a better workflow than making two surfaces and dividing?

Thanks, Dylan