=== modified file 'privatemsg/pm_block_user/pm_block_user.install'
--- privatemsg/pm_block_user/pm_block_user.install  2010-01-22 20:47:21 +0000
+++ privatemsg/pm_block_user/pm_block_user.install  2010-02-11 23:02:32 +0000
@@ -27,6 +27,7 @@
         'unsigned'      => TRUE,
       ),
     ),
+    'primary key' => array('author', 'recipient'),
   );
 
   return $schema;
@@ -39,3 +40,11 @@
 function pm_block_user_uninstall() {
   drupal_uninstall_schema('pm_block_user');
 }
+
+function pm_block_user_update_6000() {
+  $ret = array();
+  $sql = "ALTER TABLE {pm_block_user} ADD PRIMARY KEY (author, recipient)";
+  $result = db_query($sql);
+  $ret[] = array('success' => $result !== FALSE, 'query' => check_plain($sql));
+  return $ret;
+}