matlab ? Input argument "t" is undefined.

matlab ? Input argument "t" is undefined.
function x=ex_3_15(t,D)
global D;
t(find(t == 0)) = eps;
x=sin(t*pi/D)./(t*pi/D)
function ex_3_15main()
global D;
D= 2;
b1= -2;
b2= 2;
t= b1+[0:100]/100*(b2-b1);
plot(t, ex_3_15(t,D))
运行显示
? Input argument "t" is undefined.
Error in ==> ex_3_15 at 3
t(find(t == 0)) = eps;
请问怎么修改具体点谢谢