diff -u b/core/modules/block_content/block_content.install b/core/modules/block_content/block_content.install
--- b/core/modules/block_content/block_content.install
+++ b/core/modules/block_content/block_content.install
@@ -102,7 +102,7 @@
       'revision_log' => array(
         'description' => 'The log entry explaining the changes in this version.',
         'type' => 'text',
-        'not null' => TRUE,
+        'not null' => FALSE,
         'size' => 'big',
       ),
       'info' => array(