Error using mupadmex " MuPAD "

this is matlab verrrry simple program but i have verrrry simple problem :) this is the error Error using mupadmex Error in MuPAD command: DOUBLE cannot convert the input expression into a double array.

If the input expression contains a symbolic variable, use the VPA function instead.

Error in sym/double (line 936)
            Xstr = mupadmex('symobj::double', S.s, 0);

Error in vib (line 10)
nx=double( subs(x,'t',t));

and that is the program

vo=input('vo=');
xo=input('xo=');
wn=input('wn=');
syms t;
syms wn;
a1=xo;
a2=vo+(wn*xo);
x=(a1+a2*t)*exp(-wn*t);
t=0:0.5:1;
nx=double( subs(x,'t',t));
figure(1);
plot(t,nx);

what can i do to solve this program

Answer 1

[bym]

Don't define wn as symbolic

vo=input('vo=');
xo=input('xo=');
wn=input('wn=');
syms t;
%syms wn;   comment this line
a1=xo;
a2=vo+(wn*xo);
x=(a1+a2*t)*exp(-wn*t);
t=0:0.5:1;
nx=double( subs(x,'t',t));
figure(1);
plot(t,nx);