Commit 0d83783 hid a bug which should be looked at further.
This is the commit message:

The function Structure.split() returns a list of structures that result from
the splitting. In calculateBondSymmetryNumber(), this function was assumed to
return exactly two structures, which it should if things are going normally.
However, this behavior is not guaranteed, so a check was added to ensure that
exactly two structures are returned from split(). If not, the function returns
1, which is as if it was not called for that bond.