std.utf.decode behaves unexpectedly - Bug?

[HeiHon via Digitalmars-d-learn]

Consider this:

[code]
import std.stdio, std.utf, std.exception;

void do_decode(string txt)
{
     try
     {
         size_t idx;
         writeln("decode ", txt);
         for (size_t i = 0; i < txt.length; i++)
         {
             dchar dc = std.utf.decode(txt[i..i+1], idx);
             writeln(" i=", i, " length=", txt[i..i+1].length, " 
char=", txt[i], " idx=", idx, " dchar=", dc);
         }
     }
     catch(Exception e)
     {
         writeln(e.msg, " file=", e.file, " line=", e.line);
     }
     writeln();
}

void main()
{
     do_decode("abc");
/+ result:
decode abc
  i=0 length=1 char=a idx=1 dchar=a
  i=1 length=1 char=b idx=2 dchar=c
  i=2 length=1 char=c idx=3 dchar=
+/

     do_decode("åbc");
/+ result:
decode åbc
Attempted to decode past the end of a string (at index 1) 
file=D:\dmd2\windows\bin\..\..\src\phobos\std\utf.d line=1268
+/

     do_decode("aåb");
/+ result:
decode aåb
  i=0 length=1 char=a idx=1 dchar=a
core.exception.RangeError at std\utf.d(1265): Range violation
----------------
0x004054D4
0x0040214F
0x004045A7
0x004044BB
0x00403008
0x755D339A in BaseThreadInitThunk
0x76EE9EF2 in RtlInitializeExceptionChain
0x76EE9EC5 in RtlInitializeExceptionChain
+/
}
[/code]

I would expect:
decode abc -> dchar a, dchar b, dchar c
decode åbc -> dchar å, dchar b, dchar c
decode aåb -> dchar a, dchar å, dchar b

Am I using std.utf.decode wrongly or is it buggy?