DDE23 error "Derivative and history vectors have different lengths"

lags = 1;
tspan = [0 10];
sol = dde23(@ddefun, lags, @history, tspan);
function dydt = ddefun(t,y,Z) % equation being solved
  ylag1 = Z(:,1);
  global b1 N v d
  dydt = [-(b1/N)*y(1)*y(2); 
          (b1/N)*y(1)*y(2)-(v+d)*ylag1(2); 
           v*ylag1(2);
           d*ylag1(2)];
end
%-------------------------------------------
function s = history(t) % history function for t <= 0
  s = ones(4,1);
end
%-------------------------------------------

has an error message of "Error using dde23 (line 223)

Derivative and history vectors have different lengths."

[alpedhuez]

Global variables were the problem.

[Salma Al Tuwairqi]

Thank you! Your comment was a BIG help.