How to use retro over findSplitBefore result?

[MrSmith]

I am trying to compile following code

import std.algorithm : findSplitBefore;
import std.range : retro;
import std.array : array;

// returns file path where name has suffix and prefix
string withSuffixPrefix(string filePath, string prefix, string 
suffix)
{
     auto splitted = filePath.retro.findSplitBefore("/");

     return cast(string)splitted[1].retro.array
		~ prefix
		~ cast(string)splitted[0].retro.array //Fails!
		~ suffix;
}

With following error:
build.d(56): Error: template std.range.retro does not match any 
function template declaration. Candidates are:
/phobos/std/range.d(1455):   std.range.retro(Range)(Range r) if 
(isBidirectionalRange!(Unqual!Range))
build.d(56): Error: template std.range.retro(Range)(Range r) if 
(isBidirectionalRange!(Unqual!Range)) cannot deduce template 
function from argument types !()(Result)

Seems like i need to somehow convert splitted[0] to Bidirectional 
range.

This function will be used like this:
assert(withSuffixPrefix("/some/random/path/to/file", "pref-", 
".fl") == "/some/random/path/to/pref-file.fl");