Hi afni experts!

I seem to be having some trouble. I have two binary masks. One is a mask was created from fsl's FAST program and is a mask of 1s corresponding to white matter. The second mask is a ROI mask drawn for the prefrontal cortex. I was trying to use 3dcalc to get a mask that corresponds to the prefontal white matter (so essentially including only those voxels that have a value of 1 after multiplying both masks together).

I used the following command:

3dcalc -a ${subj}_Whitematter.nii -b ${subj}-PFC-dorsal-mask+acpc -expr 'a*b' -prefix ${subj}-PFC-dorsal

But this gave me the following error:
** AFNI converts NIFTI_datatype=8 (INT32) in file F0001-0708_Whitematter.nii to FLOAT32
** FATAL ERROR: dataset F0001-0708-PFC-dorsal-mask+acpc differs in size from others

I did 3dinfo on both datasets and it seems that they both have identical voxel sizes. Furthermore, since AFNI converts NIFTI to a float dataset, I converted my PFC-mask to a float dataset and still got the error.

Any help would be greatly appreciated!
Thanks in advance!