Neighbor's Solution on Old Farmer's Will

What was the problem with the will?

Neighbor went & brought a extra cow. Now there were 18 cows. The oldest son got 
1/2 x 18 = 9 cows, middle son got 1/3 x 18 = 6 cows and youngest son got 1/9 x 18 = 2 cows. In this way, total 9 + 6 + 2 = 17 cows distributed among 3 sons but left with 1 cow which neighbor took with himself.