That’s an easy one: use
dwi2response dhollander DWI.mif RF_WM.txt RF_GM.txt RF_CSF.txt -force
instead. It’s already more accurate out of the box, even on highly corrected incredible quality HCP data (see, among others, this post), but if you’re facing a scenario where you’ve got no phase-encoding information, this becomes even more of a no-brainer. I’ve seen msmt_5tt response function outcomes in such scenarios before, and it’s not pretty.