Find the bug in this program?!

lists-mindblown.py

"""
    Find all possible combinations of k numbers that add up to a number n,
    given that only numbers from 1 to 9 can be used and each combination should be a unique set of numbers.

    For example: if n = 7 k = 3
    ans [1,2,4]
    k = 3 n = 9
    [[1,2,6], [1,3,5], [2,3,4]]
"""

class CombinationArray(object):
    def __init__(self):
        return

    def combination3(self,n,k):
        ans = []
        self.result = []
        self.combinationrec(ans, k, 1, n)
        return self.result

    def combinationrec(self, ans, k, start, n):
        if n == 0 and len(ans) == k:
            self.result.append(ans)
            return

        for i in range(start, 10):
            ans.append(i)
            self.combinationrec(ans, k, i+1, n-i)
            ans.pop()


if __name__ == '__main__':
    c = CombinationArray()