Problem with map, reduce, ..

[Stefan via Digitalmars-d-learn]

I tried to refactor some existing code to use more of the 
functional patterns/style (map, filter, reduce, ..).
The task is to read in some sort of a simple property file and 
present the result as an associative array.
My attempt is:

import std.stdio;
import std.algorithm.iteration : filter, map, reduce;
import std.algorithm.mutation : split;
import std.string : indexOf, strip;
import std.stdio : File, writeln;
import std.algorithm : startsWith;

string[string] add( ref string[string] result, in string line) {
     int assignmentIndex = indexOf( line, "=" );
     if ( assignmentIndex > 0 ) {
         string key = line[ 0 .. assignmentIndex ];
         string value = line[ assignmentIndex + 1 .. line.length ];
         result[ key ] = value;
    }
    return result;
}


void main() {
    auto input = File( "test.ini", "r" );
    auto lines = input.byLine()
       .filter!( line => !startsWith( line, "#" ) && !startsWith( 
line, "!") && line.length > 0 )
       .map!( line => strip( line ) );
    string[string] props;
    auto result = reduce!(add)( props, lines );
}

result should now contain the associative array.

However, it fails with an error in the "reduce" line:

c:\develop\dmd2.067.1\windows\bin\..\..\src\phobos\std\algorithm\iteration.d(2536): Error: static assert  "Incompatible function/seed/element: prop.add/string[string]/char[]"
c:\develop\dmd2.067.1\windows\bin\..\..\src\phobos\std\algorithm\iteration.d(2521):        instantiated from here: reduceImpl!(false, MapResult!(__lambda2, FilterResult!(__lambda1, ByLine!(char, char))), string[string])
c:\develop\dmd2.067.1\windows\bin\..\..\src\phobos\std\algorithm\iteration.d(2505):        instantiated from here: reducePreImpl!(MapResult!(__lambda2, FilterResult!(__lambda1, ByLine!(char, char))), string[string])
source\prop.d(30):        instantiated from here: 
reduce!(string[string], MapResult!(__lambda2, 
FilterResult!(__lambda1, ByLine!(char, char))))

I have no idea how to resolve this.