Should we call it a bug?

IMHO the first line should evaluate OK.

Reason for Prime failing is because it automatically assigns the 0  an "appropriate" unit even though Prime is not using SUC (static unit check).  As a result, Prime naturally refuses to add 0 kg and the unitless 1.

A unitless value different from 0 works OK as can be seen in the second line.

Workaround (which should not be necessary) is using "zero" instead of "0".

There is no problem if the vector contains values of different dimensions

or even