diff --git a/core/modules/system/system.module b/core/modules/system/system.module
index 738b52e..66c9c70 100644
--- a/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -1002,6 +1002,9 @@ function _system_rebuild_module_data() {
     // Merge in defaults and save.
     $modules[$key]->info = $module->info + $defaults;
 
+    // Module description may be set but NULL. A string is required.
+    $modules[$key]->info['description'] = ($modules[$key]->info['description']) ? $modules[$key]->info['description'] : '';
+
     // Installation profiles are hidden by default, unless explicitly specified
     // otherwise in the .info.yml file.
     if ($key == $profile && !isset($modules[$key]->info['hidden'])) {