Hello,
Thank you for this hack. It is brilliant!
I remember trying to use FV with a sources file and couldn't make it work (same error as you). If I used FV and constant discharge at sources, no issues. FE for the same case worked without issue (constant or variable discharge at sources). In the end I ended up using FE, but it always bothered me why I couldn't get the FV to work.
The issue must be how the sources file is read in FV??? I am sure the development team can fix this issue promptly.
Pat