diff --git a/framework/core/src/Extend/Assets.php b/framework/core/src/Extend/Assets.php
index b09654aba..482ace057 100644
--- a/framework/core/src/Extend/Assets.php
+++ b/framework/core/src/Extend/Assets.php
@@ -15,7 +15,7 @@ use Flarum\Frontend\Event\Rendering;
 use Illuminate\Contracts\Container\Container;
 use Illuminate\Events\Dispatcher;
 
-class Assets
+class Assets implements Extender
 {
     protected $appName;
 
diff --git a/framework/core/src/Extend/Compat.php b/framework/core/src/Extend/Compat.php
new file mode 100644
index 000000000..62e059cc4
--- /dev/null
+++ b/framework/core/src/Extend/Compat.php
@@ -0,0 +1,38 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Flarum\Extend;
+
+use Illuminate\Contracts\Container\Container;
+
+/**
+ * This class is used to wrap old bootstrap.php closures (as used in versions up
+ * to 0.1.0-beta7) in the new Extender format.
+ *
+ * This gives extensions the chance to work with the new API without making any
+ * changes, and have some time to convert to the pure usage of extenders.
+ *
+ * @deprecated
+ */
+class Compat implements Extender
+{
+    protected $callback;
+
+    public function __construct($callback)
+    {
+        $this->callback = $callback;
+    }
+
+    public function __invoke(Container $container)
+    {
+        $container->call($this->callback);
+    }
+}
diff --git a/framework/core/src/Extend/Extender.php b/framework/core/src/Extend/Extender.php
new file mode 100644
index 000000000..2ee9d7e13
--- /dev/null
+++ b/framework/core/src/Extend/Extender.php
@@ -0,0 +1,19 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Flarum\Extend;
+
+use Illuminate\Contracts\Container\Container;
+
+interface Extender
+{
+    public function __invoke(Container $container);
+}
diff --git a/framework/core/src/Extend/FormatterConfiguration.php b/framework/core/src/Extend/FormatterConfiguration.php
index ad6ca0ab7..a5532ad0b 100644
--- a/framework/core/src/Extend/FormatterConfiguration.php
+++ b/framework/core/src/Extend/FormatterConfiguration.php
@@ -15,7 +15,7 @@ use Flarum\Formatter\Event\Configuring;
 use Illuminate\Contracts\Container\Container;
 use Illuminate\Events\Dispatcher;
 
-class FormatterConfiguration
+class FormatterConfiguration implements Extender
 {
     protected $callback;
 
diff --git a/framework/core/src/Extend/Locale.php b/framework/core/src/Extend/Locale.php
index 916e9da88..cff7dfab2 100644
--- a/framework/core/src/Extend/Locale.php
+++ b/framework/core/src/Extend/Locale.php
@@ -16,7 +16,7 @@ use Flarum\Locale\LocaleManager;
 use Illuminate\Contracts\Container\Container;
 use RuntimeException;
 
-class Locale
+class Locale implements Extender
 {
     protected $directory;
 
diff --git a/framework/core/src/Extend/Routes.php b/framework/core/src/Extend/Routes.php
index c02ff92eb..c867634ec 100644
--- a/framework/core/src/Extend/Routes.php
+++ b/framework/core/src/Extend/Routes.php
@@ -14,7 +14,7 @@ namespace Flarum\Extend;
 use Flarum\Http\RouteHandlerFactory;
 use Illuminate\Contracts\Container\Container;
 
-class Routes
+class Routes implements Extender
 {
     protected $appName;
 
diff --git a/framework/core/src/Extension/ExtensionManager.php b/framework/core/src/Extension/ExtensionManager.php
index a970520df..ad0df4278 100644
--- a/framework/core/src/Extension/ExtensionManager.php
+++ b/framework/core/src/Extension/ExtensionManager.php
@@ -12,6 +12,7 @@
 namespace Flarum\Extension;
 
 use Flarum\Database\Migrator;
+use Flarum\Extend\Compat;
 use Flarum\Extension\Event\Disabled;
 use Flarum\Extension\Event\Disabling;
 use Flarum\Extension\Event\Enabled;
@@ -285,7 +286,14 @@ class ExtensionManager
                 if ($this->filesystem->exists($bootstrapper)) {
                     $extenders = require $bootstrapper;
 
-                    return (array) $extenders;
+                    if (is_array($extenders)) {
+                        return $extenders;
+                    }
+
+                    // Assume that the extension has not yet switched to the new
+                    // bootstrap.php format, and wrap the callback in a Compat
+                    // extender.
+                    return [new Compat($extenders)];
                 } else {
                     return [];
                 }