Unicode exception raise when replacing underscore with space

["Nordlöw" via Digitalmars-d-learn]

I get

     core.exception.UnicodeException at src/rt/util/utf.d(290):

in a call to

     std.string.tr(x, `_`, ` `)

for a badly encode string x. Is it really needed to do 
auto-decoding here?

Isn't the encoding of underscore and space uniquely one by byte 
in UTF-8?

What do I need to do/add to avoid auto-decoding here?