Function 'subsindex' is not defined for values of class 'tf'

I have been trying to work on GSO code, and it keeps showing this error I made sure there was no variable in any function called step. Does anyone point me towards what might be wrong?

Function 'subsindex' is not defined for values of class 'tf'.

Error in UpdateLuciferin (line 28) J(i,:)=k(2);

Error in GSO (line 34) UpdateLuciferin; % Update the luciferin levels at glowworms’

    function UpdateLuciferin
global n A J Ell gamma ro
for i = 1 : n
T = A(i,1);
K = A(i,2);
G=[0 0 0 1 0 0 0 0 0;0 0 0 0 1 0 0 0 0;0 0 0 0 0 1 0 0 0;-0.003184 0 0 0 0 -0.0199 0 0 0.0199;0 0.0002985 0 0 0 0 0 0 0;0 0 -0.000398 0.00997 0 0 -0.00997 0 0;0 0 0 0 0 0 0 0 0;0 0 0 0 0 0 0 0 0;0 0 0 0 0 0 0 0 0];
B=[0 0 0;0 0 0;0 0 0;-1 0 0;0 -0.25 0;0 0 -0.5;1 0 0;0 1 0;0 0 1];
C=[1 0 0 0 0 0 0 0 0;0 1 0 0 0 0 0 0 0;0 0 1 0 0 0 0 0 0];
D=0;
sys = ss(G,B,C,D);
i=tf(sys);
X=i(1,1);
Y=i(1,2);
Z=i(1,3);
num=[(0.6*(T^2)*K) 4.8*K*T 9.6*K];
den=[8*T 0];
tfPID=tf(num,den);
xtf=X*tfPID;
ytf=Y*tfPID;
ztf=Z*tfPID;
a=xtf/(1+xtf);
b=ytf/(1+ytf);
c=ztf/(1+ztf);
ftf=a+b+c;
y=step(ftf);
yinfo=stepinfo(y)
k=struct2cell(yinfo);
J(i,:)=k(2);
% The Matlab ’Peaks’ function is used here. Please replace it with
% the multimodal function for which peaks are sought
%J(i,:) =-(647.9-0.7276.*x+0.6765.*y-0.1455.*x.^2+0.2327.*x.*y-0.17.*y.^2+0.001928.*x.^3-0.003643.*x.^2.*y+0.00255.*x.*y.^2-0.0008596 .*y.^3+0.0001613.*x.^4-0.0002075.*x.^3.*y+(1.244e-05).*x.^2.*y.^2+(9.413e-06).*x.*y.^3+0.0001003.*y.^4);
    Ell(i,:) = (1-ro)*Ell(i,:) + gamma*J(i,:);
end

[Steven Lord]

This is a duplicate question.