diff --git a/config/configuration.js b/config/configuration.js
index d191483..7c9034a 100644
--- a/config/configuration.js
+++ b/config/configuration.js
@@ -7,5 +7,5 @@ module.exports = {
   aliases: require('../config/json/aliases.json'),
   documentRelatedEndpoints: require('../config/json/document-related-endpoints.json'),
 
-  apiHost: 'https://api.anyfetch.com'
+  apiHost: process.env.API_HOST || 'https://api.anyfetch.com'
 };
\ No newline at end of file
diff --git a/config/json/document-related-endpoints.json b/config/json/document-related-endpoints.json
index a2f4e8c..88d715c 100644
--- a/config/json/document-related-endpoints.json
+++ b/config/json/document-related-endpoints.json
@@ -1,6 +1,14 @@
 {
-  "getSimilar": { "endpoint": "/documents/{id}/similar" },
-  "getRelated": { "endpoint": "/documents/{id}/related" },
-  "getRaw": { "endpoint": "/documents/{id}/raw" },
-  "getFile": { "endpoint": "/documents/{id}/file" }
+  "getSimilar": {
+    "endpoint": "/documents/{id}/similar"
+  },
+  "getRelated": {
+    "endpoint": "/documents/{id}/related"
+  },
+  "getRaw": {
+    "endpoint": "/documents/{id}/raw"
+  },
+  "getFile": {
+    "endpoint": "/documents/{id}/file"
+  }
 }
\ No newline at end of file
diff --git a/lib/index.js b/lib/index.js
index dfee51f..17fcab8 100644
--- a/lib/index.js
+++ b/lib/index.js
@@ -14,9 +14,15 @@ var accessToken = process.env.ACCESS_TOKEN;
 
 var anyfetchRequest = request(configuration.apiHost);
 
+/**
+ * Determine if `f` is a function
+ */
+var isFunction = function(f) {
+  return !!(f && f.constructor && f.call && f.apply);
+};
+
 /**
  * Will be used to automatically generate the simplest mapping functions
- * @see api.anyfetch.com/config/routes.js
  */
 var generateMapping = function(config, name) {
   // `config` extends the default descriptor
@@ -37,13 +43,8 @@ var generateMapping = function(config, name) {
    * apiMapping(params, cb)
    */
   return function apiMapping() {
-    var argErrorMessage = 'Argument error in ' + name + ':';
-
     // Convert arguments to an array
-    var args = [];
-    for (var i in arguments) {
-      args.push(arguments[i]);
-    }
+    var args = Array.prototype.slice.call(arguments);
 
     // We must support quite a lot of different call syntaxes
     var params = {};
@@ -52,8 +53,8 @@ var generateMapping = function(config, name) {
     // cb is always the last argument
     var cb = args.pop();
     // cb must be a function
-    if (!(cb && cb.constructor && cb.call && cb.apply)) {
-      throw new Error(argErrorMessage + ' the last argument must be a function (callback)');
+    if (!isFunction(cb)) {
+      throw new Error('Argument error in ' + name + ':' + ' the last argument must be a function (callback)');
     }
 
     if (config.body) {
@@ -61,7 +62,7 @@ var generateMapping = function(config, name) {
       // Make sure that every field of the body is accepted
       for(var key in body) {
         if(config.body.indexOf(key) === -1) {
-          return cb(new Error(argErrorMessage + ' the key ' + key + ' is not allowed in this request\'s body'));
+          return cb(new Error('Argument error in ' + name + ':' + ' the key ' + key + ' is not allowed in this request\'s body'));
         }
       }
     }
@@ -71,7 +72,7 @@ var generateMapping = function(config, name) {
       // Make sure that every GET parameter is accepted
       for(var key in params) {
         if(config.params.indexOf(key) === -1) {
-          return cb(new Error(argErrorMessage + ' the key ' + key + ' is not allowed in this request\'s GET parameters'));
+          return cb(new Error('Argument error in ' + name + ':' + ' the key ' + key + ' is not allowed in this request\'s GET parameters'));
         }
       }
     }
@@ -80,7 +81,7 @@ var generateMapping = function(config, name) {
     if(config.requireId) {
       var id = args.pop();
       if(isNaN(id)) {
-        return cb(new Error('Argument error, the first parameter must be a numeric id'));
+        return cb(new Error('Argument error in ' + name + ':' + ' the first parameter must be a numeric id'));
       }
 
       // Substitute id parameter in endpoint string
@@ -141,38 +142,52 @@ var generateAliases = function(mappingFunctions) {
 var generateAdvancedMappings = function(mappingFunctions) {
   // ----- Documents
   var getDocumentByIdCb = mappingFunctions.getDocumentById;
-  mappingFunctions.getDocumentById = function() {
-    // Invalid call syntax
-    if (arguments.length > 2 || isNaN(arguments[0])) {
-      // We assume the last argument is a callback
-      arguments[arguments.length - 1](new Error('Invalid call syntax'));
-    }
-    var id = arguments[0];
+
+  /**
+   * Support two call syntaxes:
+   * - Single step: getDocumentById(123, cb)
+   * - Two steps: getDocumentById(123).getRelated(cb)
+   */
+  mappingFunctions.getDocumentById = function(id, cb) {
 
     // Single-step call syntax
-    // getDocumentById(123, cb)
-    if (arguments.length === 2) {
+    if (isFunction(cb)) {
+      if (isNaN(id)) {
+        return cb(new Error('Argument error in getDocumentById: the first argument must be a numeric id'));
+      }
       return getDocumentByIdCb(id, arguments[1]);
     }
 
     // Two-step call syntax
-    // getDocumentById(123).getRelated(cb)
     var secondStep = {};
     for (var name in configuration.documentRelatedEndpoints) {
-      var opt = {
-        endpoint: configuration.documentRelatedEndpoints[name].endpoint.replace('{id}', id)
+      var opt = configuration.documentRelatedEndpoints[name];
+      // Trick: "hardcode" id parameter in endpoint and proceed as usual
+      opt.endpoint = opt.endpoint.replace('{id}', id);
+
+      secondStep[name] = function(){
+        if (isNaN(id)) {
+          // We assume the last parameter is a callback
+          var cb = arguments[arguments.length - 1];
+          return cb(new Error('Argument error in getDocumentById: the first argument must be a numeric id'));
+        }
+
+        generateMapping(opt, name).apply(this, arguments);
       };
-      secondStep[name] = generateMapping(opt, name);
     }
 
-      /**
-       * Add a file to a previously uploaded document
-       *
-       * @param {Object} Must at least contain a `file` key, which can either be a stream (e.g. `fs.createReadStream`) or a Buffer object. Can also contains a `contentType` key (for MIME type), and a `filename`.
-       * @param {Function} cb Callback with error if any.
-       * @Warning: unfortunately, due to the variety of Stream, we can't type-check, so unexpected errors will occur if you specify weird file parameters.
-       */
+    /**
+     * Add a file to a previously uploaded document
+     *
+     * @param {Object} Must at least contain a `file` key, which can either be a stream (e.g. `fs.createReadStream`) or a Buffer object. Can also contains a `contentType` key (for MIME type), and a `filename`.
+     * @param {Function} cb Callback with error if any.
+     * @Warning: unfortunately, due to the variety of Stream, we can't type-check, so unexpected errors will occur if you specify weird file parameters.
+     */
     secondStep.postFile = function(config, cb) {
+      if (isNaN(id)) {
+        return cb(new Error('Argument error in getDocumentById: the first argument must be a numeric id'));
+      }
+
       if(typeof(config) === 'function') {
         config = config();
       }