Dear Josh

thanks so much for spotting this.
We fixed the knee_pt function such that it now returns the minimum when the input vector has less than three elements, and no longer an error message.

The fix is done in the matlab repo, the python one will be updated very soon (I want to check with the python developer that the pypi package and the docker container are uploaded consistently).

As you can imagine, the estimation (in particular the width and time to peak) will be less accurate for long TRs.
You can also try to increase the lag search interval a bit (both sides of the interval). The [4 8] was actually optimized for short TRs.

thanks again!