Bug in countUntil?

[monarch_dodra]

I was looking in countUntil to fix another issue, and I think the 
string support is broken

This program:
//----
import std.algorithm;
import std.stdio;

void main()
{
     "日本語".countUntil('本').writeln();
}
//----

Will produce "3".

...

I'd have straight up said it was a bug, but the implementation 
goes out of its way to special case narrow strings, when the 
default implementation would have produced the right result 
anyway. So I was thinking it is somehow by design...?

Am I missing something, or is it just implementation sillyness?