diff --git a/core/modules/block/block.module b/core/modules/block/block.module
index a973f8f..c5220af 100644
--- a/core/modules/block/block.module
+++ b/core/modules/block/block.module
@@ -127,12 +127,13 @@ function block_theme_initialize($theme) {
       // theme's default region.
       if (!isset($regions[$block->getRegion()])) {
         $info = system_get_info('theme', $theme);
-        if (isset($info['block_default_region'])) {
+        if (isset($info['block_default_region'], $regions[$info['block_default_region']])) {
         $block->setRegion($info['block_default_region']);
         }
         else {
         $block->setRegion(system_default_region($theme));
         }
+      }
       $block->save();
     }
   }