nrbderiv resets the nurbs weight to zero in build-in bspderiv. nrbkntplot would not plot a nurbs with zero weights. Please see the attached ofig.  Thanks. 

pkg load nurbs
Q=[0, 0; 3,4;-1,4;-4,0;-4,-3]
p=3; % cubic
%
P=[0        0; ...
   7.3170   3.6868; ...
  -2.9581   6.6783; ...
  -4.4950  -0.6737; ...
  -4.0000  -3.0000];
U=[0        0        0        0   0.5490   1.0000   1.0000   1.0000   1.0000];
nurbs   = nrbmak(cp=P',knots=U);
nurbs_d = nrbderiv (nurbs);nurbs_d.coefs  % zero weight

figure('name', 'nrbderiv weight=0');
 
subplot(1,2,1); % no plot
nrbkntplot(nurbs_d);  

subplot(1,2,2); % plot
nurbs_d.coefs(4,:)=[1 1 1 1];
nrbkntplot(nurbs_d);