Question: problem with large calculation

I have a problem with my calculations. I have changed the Order to 15. If I go any higher than this I get the error message:

"Error, invalid input: rhs expects 1 argument, but received 0"
 

after the loop. I need to calculate more terms. Does anyone know why this is happening and how I can fix it?

This is the spreadsheet

> restart;
> with(plots);
> nmax:=14:
> Order:=nmax+1:

Start with the differential equation
> de:=diff(F(eta,R),eta$4)-R*(diff(F(eta,R),eta)*diff(F(eta,R),eta$2)-F(eta,R)*diff(F(eta,R),eta$3)):

Write in a series of powers of R
> series(eval(de,F(eta,R)=add(F[k](eta)*R^k,k=0..nmax)),R):

Extract the individual equations for each power upto R^5
> des:=[seq(coeff(%,R,k),k=0..nmax)]:

Define the boundary conditions
> bcs:=k->(F[k](-1)=`if`(k=0,-1,0),F[k](1)=`if`(k=0,1,0),D(F[k])(-1)=0,D(F[k])(1)=0):

Solve the boundary value problem for each power
> for k from 1 to nmax+1 do
>     F[k-1]:=unapply(rhs(dsolve({des[k],bcs(k-1)})),eta);
> end do:
 

Any help would be much appreciated. Thanks