help! where a function was expected.

hi:
I have a script but some thing ,solve ode use series.
rightfun(x):=taylor(sin(x),x,0,5);
yp(x):=diff(ys(x),x);
ydp(x):=diff(ys(x),x,2);
eqn1:ydp(x)+2*yp(x)+ys(x)-sin(x)=0;
c:[1,0,-1/2,1/2,-5/24,1/20];
ys(x):=sum(c(n)*x^n,n,0,5);
sequ:lhs(eqn1);
subst(seqn,makelist(c(i),i,0,5));

an error is:
 apply: found c evaluates to [1,0,-1/2,1/2,-5/24,1/20] where a function was
expected.
 what's matter
--elan