HMAC and toHexString

[Nicholas Wilson]

...
string key = "blahblahblah";
auto mac = hmac!SHA256(key.representation);
string s = ...,t=...u=...,v=...;
foreach(w;AliasSeq!(s,t,u,v))
             mac.put(w.representation);
ubyte[32] s = mac.finish;
string sig = toHexString!(LetterCase.lower)(s);

writeln(sig);
// From what I understand Should print something like 
41771e2d0d6c1cf7f442aa8547783ea5b126bfc15a4b354e94d0eaea2ab0fa1a
// Actually prints something like x"31 36 39 33 39 32 38 31 62 38 
31 62 36 36 62 63 63 34 63 36 36 61 62 32 34 37 64 32 64 34 61 00 
78 2A 55 5B FF 7F 00 00 78 2A 55 5B FF 7F 00 00 E0 2A 55 5B FF 7F 
00 00 08 2C 55 5B FF 7F 00 00"c
Note the nuls and things like FF

What am I doing wrong here?