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Algorithm for listing all solutions to a linear Diophantine equation.

Consider the linear Diophantine equation of the form: $$\sum_{i=1}^{k}a_ix_i=n.$$ My goal is to list all the non-negative solutions to this equation. I am trying to write a recursive algorithm, but I am not able to build the solution recursively. Here is what I have so far:

import math
def ListOfSolutions(n,L):
 if len(L) == 1:
    ak = L[0]
    if n%ak == 0:
        return [n//ak]
        return [None]
 a1 = L[0]
 ans = []
 end = math.floor(n/a1)
 for k in range(0, end + 1):
     res = ListOfSolutions(n-k*a1,L[1:])
     if None not in res:
         ans.append([k] + res)
 return ans

print (ListOfSolutions(5,[2,1]))

The algorithm works for a list of two coefficients $[2,1]$ but when I try $3$ or more I get errors. Perhaps someone could give me some indication as to what I am doing wrong here.