utf8.len(str) == nil

[johnd0e]

Many routines are the same as Lua's string module:

• ...
• utf8.len

In fact I see the difference when processing invalid utf8 strings:

local a = "\128\129\130"
print(utf8.len(a))

nil    1

This difference can lead to unobvious bugs.

So it must be either documented, or changed to throw error.