String bound check allow to access last null char?

[Ward]

Consider this code: 
    
    
    var a = @[1, 2, 3]
    echo a[a.len]
    
    
    Run

Of course we will get an IndexError: "index out of bounds". However, this code 
will pass. 
    
    
    var s = "123"
    echo s[s.len]
    
    
    Run

We even can modify the last null char. Is this a bug or a default behavior? 
Anyway, I think it's not safe.