diff --git a/classes/File_thumbnail.php b/classes/File_thumbnail.php
index 925a3b2429..3f36e26563 100644
--- a/classes/File_thumbnail.php
+++ b/classes/File_thumbnail.php
@@ -134,7 +134,8 @@ class File_thumbnail extends Managed_DataObject
             throw new ClientException(_('Invalid filename.'));
         }
 
-        $dir = common_config('thumbnail', 'dir') ?: File::path('thumb');
+        // NOTE: If this is empty, it will be set to File::path('thumb')
+        $dir = common_config('thumbnail', 'dir');
 
         if (!in_array($dir[mb_strlen($dir)-1], ['/', '\\'])) {
             $dir .= DIRECTORY_SEPARATOR;
diff --git a/lib/gnusocial.php b/lib/gnusocial.php
index f07d2c2446..3450b04888 100644
--- a/lib/gnusocial.php
+++ b/lib/gnusocial.php
@@ -141,6 +141,7 @@ class GNUsocial
         // Load settings from database; note we need autoload for this
         Config::loadSettings();
 
+        self::fillConfigVoids();
         self::verifyLoadedConfig();
 
         self::initPlugins();
@@ -420,6 +421,14 @@ class GNUsocial
         }
     }
 
+    static function fillConfigVoids()
+    {
+        // special cases on empty configuration options
+        if (!common_config('thumbnail', 'dir')) {
+            common_config_set('thumbnail', 'dir', File::path('thumb'));
+        }
+    }
+
     /**
      * Verify that the loaded config is good. Not complete, but will
      * throw exceptions on common configuration problems I hope.