diff --git a/modules/taxonomy/taxonomy.module b/modules/taxonomy/taxonomy.module
index c920344..d06e8c2 100644
--- a/modules/taxonomy/taxonomy.module
+++ b/modules/taxonomy/taxonomy.module
@@ -568,7 +568,7 @@ function taxonomy_check_vocabulary_hierarchy($vocabulary, $changed_term) {
       $hierarchy = 2;
       break;
     }
-    elseif (count($term->parents) == 1 && 0 !== array_shift($term->parents)) {
+    elseif (count($term->parents) == 1 && !isset($term->parents[0])) {
       $hierarchy = 1;
     }
   }
diff --git a/modules/taxonomy/taxonomy.test b/modules/taxonomy/taxonomy.test
index 461f277..bca75ca 100644
--- a/modules/taxonomy/taxonomy.test
+++ b/modules/taxonomy/taxonomy.test
@@ -560,6 +560,10 @@ 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, 'Vocabulary is flat.');
+
     // Edit $term2, setting $term1 as parent.
     $edit = array();
     $edit['parent[]'] = array($term1->tid);