conver BigInt to string

[Namal via Digitalmars-d-learn]

Hello I am trying to convert BigInt to string like that while 
trying to sort it:

string s1 = to!string(a).dup.sort;
and get an error

cannot implicitly convert expression (_adSortChar(dup(to(a)))) of 
type char[] to string

what do I do wrong?