Hi there, I just played around a bit with calculating derivatives of parametric curves keeping the order as a variable and found a strange behaviour. When typing

line[t_] := {p1 + a t, p2 + b t}
D[line[t], {t,0}]

the output is

{p1 + a t, p2 + b t}

as one would expect.

However, when typing

D[line[t], {t,k}]

with variable k, then the result is

so in case of k=0, the constant terms p1 and p2 are missing. Maybe I don't understand it right, but it looks like a bug to me, right?

BTW: I'm using Wolfram Language 12 (Linux) but the behaviour is the same in Wolfram Cloud.

Best regards Volker