range code doesn't work for multi-array

[AlphaPurned via Digitalmars-d-learn]

if arr is a single array the follow code works(removing [i]) but 
if it is a double array it fails. arr as type double[A][B]. 
arr[i] has type double[A].
v ~= arr[i].map!(a=>to!string(a)~",").join;

  with `Range = double[21]`
  must satisfy the following constraint:
`       isInputRange!(Unqual!Range)`