# V8031\. Using the call of the 'len' function as an index will result in an off\-by\-one error\.

The analyzer has detected a potential off\-by\-one error caused by using the result of the len function as an index\. Such an error causes the program to panic\.

The code example:

```cpp
func End(basicLits []*BasicLit) token.Pos { 
  return basicLits[len(basicLits)].End()
}
```

Developers wanted to obtain the last element position of the `basicLits` slice\. However, for a `n`\-length slice, valid indices range from `0` to `n - 1`, so accessing `basicLits[len(basicLits)]` always results in the program panic\.

Fixed code:

```cpp
func End(basicLits []*BasicLit) token.Pos {
  if length := len(basicLits); length > 0 { 
    return basicLits[length - 1].End()
  }
  return token.NoPos
}
```

If a function has a precondition of working with non\-zero length slices, then the `length > 0` check can be removed\.