diff --git a/election_vote/election_vote.install b/election_vote/election_vote.install
index f73792c..5186456 100644
--- a/election_vote/election_vote.install
+++ b/election_vote/election_vote.install
@@ -175,9 +175,6 @@ function election_vote_schema() {
       'abstain' => array('abstain'),
       'value' => array('value'),
     ),
-    'unique keys' => array(
-      'one_vote_per_person' => array('post_id', 'uid'),
-    ),
     'foreign keys' => array(
       'election' => array(
         'table' => 'election',
@@ -267,16 +264,26 @@ function election_vote_update_7003() {
 }
 
 /**
- * Add unique index to prevent multiple votes.
+ * Blank update, left for backwards compatibility.
  */
 function election_vote_update_7004() {
-  db_add_unique_key('election_ballot', 'one_vote_per_person', array('election_id', 'post_id', 'uid'));
+  // This update added the one_vote_per_person unique key, but the key is
+  // removed in a later update (7006).
 }
 
 /**
- * Improve unique index to prevent multiple votes.
+ * Blank update, left for backwards compatibility.
  */
 function election_vote_update_7005() {
-  db_drop_unique_key('election_ballot', 'one_vote_per_person');
-  db_add_unique_key('election_ballot', 'one_vote_per_person', array('post_id', 'uid'));
+  // This update modified the one_vote_per_person unique key, but the key is
+  // removed in a later update (7006).
+}
+
+/**
+ * Remove the one_vote_per_person unique index (see https://drupal.org/node/2627710 for more detail).
+ */
+function election_vote_update_7006() {
+  if (db_index_exists('election_ballot', 'one_vote_per_person')) {
+    db_drop_unique_key('election_ballot', 'one_vote_per_person');
+  }
 }