Fixed getting alternatives

src/Plugin.php

@@ -200,8 +200,10 @@ protected function processPackage(Package $package)
             $alternatives = $package->getAlternatives();
             if (is_string($alternatives)) {
                 $this->alternatives = $this->readConfig($package, $alternatives);
-            } else {
+            } elseif (is_array($alternatives)) {
                 $this->alternatives = $alternatives;
+            } elseif (!empty($alternatives)) {
+                throw new BadConfigurationException('alternatives must be array or path to configuration file');
             }
         }
 

src/exceptions/BadConfigurationException.php

@@ -0,0 +1,20 @@
+<?php
+/**
+ * Composer plugin for config assembling
+ *
+ * @link      https://github.com/hiqdev/composer-config-plugin
+ * @package   composer-config-plugin
+ * @license   BSD-3-Clause
+ * @copyright Copyright (c) 2016-2018, HiQDev (http://hiqdev.com/)
+ */
+
+namespace hiqdev\composer\config\exceptions;
+
+/**
+ * Bad configuration exception.
+ *
+ * @author Andrii Vasyliev <sol@hiqdev.com>
+ */
+class BadConfigurationException extends Exception
+{
+}