That’s weird. I must say I haven’t used that script in years since I’m not a Matlab user anymore, so I’m a bit in the dark on its internals. It looks like the Matlab function either has trouble reading the file header (which would then also be exceptionally long, seeing how it mentions line 26), or is somehow tricked into thinking the streamlines are encoded in ascii (while they should be binary floats). Are you sure your .tck file is uncorrupted (check in mrview for instance)? If it’s fine, you may want to step through the function line by line with the Matlab debugger to find out at which point the error occurs.