[luatex] accented characters in plain luatex

[Bob Tennent]

I'm finding that accented characters (à, é, ä, etc.) in my documents
produce

! String contains an invalid utf-8 sequence.

But the manual says

  All LuaTeX text input and output is considered to be Unicode text.

Is there a simple fix, apart from resorting to \`a, \'e, \:a, etc.?

Bob T.