[NTG-context] A question about whatever (Metapost)

Hi, The point M is such that the line (CM) is perpendicular to the line (NP). I do not understand the error of this code. Thank you Fabrice \usecolors[xwi] \starttext \startMPcode path carre, p; carre = unitsquare scaled 138; pair a, b, c, d, N, P, M; a = point 0 of carre; b = point 1 of carre; c = point 2 of carre; d = point 3 of carre; p = b -- d; N = whatever[a,d]; P = whatever[a,b]; M = whatever[b,d]; (M - c) dotprod (P - N) = 0; draw M -- c; draw p withcolor \MPcolor{navy}; draw carre withcolor \MPcolor{firebrick}; label.lft("$A$", point 0 of carre); label.lrt("$B$", point 1 of carre); label.urt("$C$", point 2 of carre); label.ulft("$D$", point 3 of carre); \stopMPcode \stoptext