Odd behavior with pairs on string and runeAt

[TotalKnowledge]

Perhaps I am doing something wrong (just learning nim) but the following code 
is outputting "extra" characters

> var str = "u5f7cu884cu3067"
> 
> for key, value in pairs(str):
>     echo runeAt(str, key)

which outputs:

> 彼

½ ¼ 行 ¡ Œ で  §