Error in acker function, poles do not much size A?

I am trying to use the ACKER function in my code but i keep getting an error and not sure why. The syntax seems to be correct so I am not sure what is wrong with it. I seems to start with the function on line 26 but refers to the error being in line 31?

clc; 
close all;

s = tf("s");
Mg = 5.7; Ms = 6.7; Jm = 10.91e-3; 
r = 31.75e-3;
b = 0.268;
Km = 0.8379; Kb = 0.838; Ra = 1.36; La = 3.5e-3;
JT = Jm + (r^2)*(Mg+Ms);
Gload = 1/((JT*s+b));
Garm = Km/(La*s+Ra);
G = feedback(Gload*Garm, Kb);
G_dcmotor = minreal(G/s);

%requirements 
Mp = 2/100; 
Ts = 100e-3;
zeta = -log(Mp)/sqrt(pi^2+(log(Mp))^2);
wn = 4/(zeta*Ts);
wd = wn * sqrt(1-zeta^2);
poles = (-zeta*wn+1i*wd-zeta*wn-1i*wd-100*zeta*wn);
 
% Required Gain Matrix
[num,den] = tfdata(G_dcmotor);
[A,B,C,D] = tf2ss(num{1},den{1});
Gain = acker(A,B,poles); % this is where it says the error is.

%New State Matrix
A_new = A-B*Gain; % error refers back to this line and says pole do not match size (A)?
B_new = B; 
C_new = C; 
D_new = 0; 
sys = ss(A_new, B_new, C-new, D_new);
dc_gain = dcgain(sys);
N = 1/(dc_gain);  % Scaling Factor
sys_mod = ss(A_new, N*B_new, C_new, D_new); 
step(sys_mod); 
stepinfo(sys_mod);

Answer 1

[Paul]

poles should have three elements based on the size of A, but it only has one:

>> poles

poles =

  -4.0800e+03