Strange error when compiling with dmd, not with ldc

[Fra Mecca]

I have this struct:

immutable struct Configuration {
     string title;
     string baseurl;
     string url;
     string email;
     string author;
     string parser;
     string target;
     string urlFormat;
     string urlFormatCmd;

     short port;

     string[] ignore;
     string[] extensions;

     @property string toString()
     {
         auto urlF = (urlFormatCmd ? "url_format_cmd: " ~ 
urlFormatCmd : "") ~ "\n";
         return
         "title: "        ~ title ~ "\n" ~
         "baseurl: "      ~ baseurl ~ "\n" ~
         "url: "          ~ url ~ "\n" ~
         "email: "        ~ email ~ "\n" ~
         "author: "       ~ author ~ "\n" ~
         "parser: "       ~ parser ~ "\n" ~
         "target: "       ~ target ~ "\n" ~
         "url_format: "   ~ urlFormat ~ "\n" ~
         "ignore: "       ~ to!string(ignore)[1 .. $ - 1] ~ "\n" ~
         "extensions: "   ~ to!string(extensions)[1 .. $ - 1] ~ 
"\n" ~
         urlF;
     }
}

and this function:

void show_config()
{
     writef("%s", parse_config(
                 exists("config.sdl") ? "config.sdl" : 
"").toString);
}


Whenever I compile with ldc2 I get no errors, while with dmd I 
get:

source/configuration.d(105,27): Error: immutable method 
configuration.Configuration.toString is not callable using a 
mutable object


What is the problem?