how to solve this error?

??? Error using ==> lti.subsref at 56
Use two or more subscripts to select or delete all elements,
as in the "sys(:,:)" command.

Error in ==> conv at 40
c = conv2(a(:),b(:),shape);

Error in ==> secondary at 14
d1=conv(s,y);
my code is
function x=signal(f,fs,N)
% sinusoid
f=30;    %frequency, 
N=20;    %number of samples
fs=100;   %sampling frequency
A=2;      %amplitute 
%Total sampling time = 1 second
n = 0:N-1; % numbering the samples
x = A*sin(2*pi*f*n/fs);
w=0.3;
[b,a]=butter(N,w);
y = filter(b,a,x);
s=tf([0.4923 0.96 1 0],[0.3352 1.06 1]);
d1=conv(s,y);
ff_sec = abs(fft(d1,1024));
subplot(2,1,1);
plot(d1)
subplot(2,1,2);
plot(ff_sec,'m');