When you recursively call sum_series here, you're doing it without any other parameters, so it's defaulting to the fibonacci parameters. This means your sum_series will only work with the two series you're explicitly checking for (lucas and fibonacci). If you structure this call like you did in the other two series (return sum_series(k-1...) then it should work. The reason your tests are passing is that they're only checking for fibonacci and lucas series numbers. So if I put in sum_series(3, 3, 3) I should get out 6, as 3+3 = 6. Does this make sense?