[NTG-context] Problem to place a label in an environment \startMPcode \stopMPcode

Hello, The code below works perfectly without the placement of the label, but if I want to place the label, it no longer works. Thank you \definefloat [recipe] [figure] \setupfloat [recipe] [default={right,none}] \starttext \switchtobodyfont [pagella, 9pt] \placerecipe{}{ \startMPcode pair vert[] ; n:=6 ; radius=2cm ; for i=0 upto n : vert[i] = radius * dir(360*i/n) ; endfor ; draw for i=0 upto n-1: vert[i] -- endfor cycle withcolor blue ; draw for i=0 upto n-1: vert[i] .. endfor cycle withcolor red ; draw (0,0)--vert[0] withcolor green ; draw (0,0)--vert[5] withcolor green ; draw (0,0) --(vert[0]+vert[5])/2 dashed evenly ; % label.urt("C",vert[1]) ; \stopMPcode } \input knuth \stoptext