OEIS A228297: Need Help to Find Algorithm

[Gilles59]

Code:

« -> n
  «
    CASE 
      'n<=s+1' THEN 1 END 
      'n<=s+k' THEN n s - END 
      `∑(j=1,k,a(n-j-(s-1)-a(n-j)))`
    END
  »
»
'a' STO

With 0 's' STO , 5 'k' STO
I get correct result for a(1) to a(5)

but a(6) returns "THEN error :undefined name"

What is the problem ?