Concatenation of ubyte[] to char[] works, but assignation doesn't

[skilion via Digitalmars-d-learn]

Is this allowed by the language or it is a compiler bug ?
void main() {
   char[] a = "abc".dup;
   ubyte[] b = [1, 2, 3];
   a = b;   // cannot implicitly convert expression (b) of type 
ubyte[] to char[]
   a ~= b;  // works
}