diff --git a/core/modules/taxonomy/taxonomy.module b/core/modules/taxonomy/taxonomy.module
index 719817b..e82edfc 100644
--- a/core/modules/taxonomy/taxonomy.module
+++ b/core/modules/taxonomy/taxonomy.module
@@ -517,7 +517,7 @@ function taxonomy_check_vocabulary_hierarchy(TaxonomyVocabulary $vocabulary, $ch
       $hierarchy = TAXONOMY_HIERARCHY_MULTIPLE;
       break;
     }
-    elseif (count($term->parents) == 1 && 0 !== array_shift($term->parents)) {
+    elseif (count($term->parents) == 1 && !isset($term->parents[0])) {
       $hierarchy = TAXONOMY_HIERARCHY_SINGLE;
     }
   }
diff --git a/core/modules/taxonomy/taxonomy.test b/core/modules/taxonomy/taxonomy.test
index 65d6805..dfd14bc 100644
--- a/core/modules/taxonomy/taxonomy.test
+++ b/core/modules/taxonomy/taxonomy.test
@@ -584,6 +584,10 @@ class TaxonomyTermTestCase extends TaxonomyWebTestCase {
     $term1 = $this->createTerm($this->vocabulary);
     $term2 = $this->createTerm($this->vocabulary);
 
+    // Check that hierarchy is flat.
+    $vocabulary = taxonomy_vocabulary_load($this->vocabulary->vid);
+    $this->assertEqual(0, $vocabulary->hierarchy, t('Vocabulary is flat.'));
+
     // Edit $term2, setting $term1 as parent.
     $edit = array();
     $edit['parent[]'] = array($term1->tid);