The problem may be in the number of bonus balls there are. You did not mention how many there are to choose from. 1 is chosen from n number of bonus balls.
In the US there is a similar lottery called Megamillions.
There are 5 balls chosen from 56 and 1 bonus from 46.
Let's say in this mega game we want the prob. of choosing 4 out of 5 numbers and no mega ball. That is like yours only you have 5 out of 6.
We choose 4 of the 5 good balls and 1 from the remaining 51 loser balls. We are also choosing 1 of the 45 loser mega balls. I believe this may be your problem.
Check to see how many bonus balls there are in your game. You are not matching it, so you want to choose C(n-1,1). Where n is the number of loser bonus balls.