diff --git a/src/hb-priority-queue.hh b/src/hb-priority-queue.hh
index 9c02eb423..b3c7ed84f 100644
--- a/src/hb-priority-queue.hh
+++ b/src/hb-priority-queue.hh
@@ -63,9 +63,11 @@ struct hb_priority_queue_t
 
   item_t pop_minimum ()
   {
-    item_t result = heap[0];
+    assert (!is_empty ());
 
-    heap[0] = heap[heap.length - 1];
+    item_t result = heap.arrayZ[0];
+
+    heap.arrayZ[0] = heap.arrayZ[heap.length - 1];
     heap.shrink (heap.length - 1);
     bubble_down (0);
 
@@ -104,6 +106,8 @@ struct hb_priority_queue_t
 
   void bubble_down (unsigned index)
   {
+    assert (index <= heap.length);
+
     unsigned left = left_child (index);
     unsigned right = right_child (index);
 
@@ -113,11 +117,11 @@ struct hb_priority_queue_t
       return;
 
     bool has_right = right < heap.length;
-    if (heap[index].first <= heap[left].first
-        && (!has_right || heap[index].first <= heap[right].first))
+    if (heap.arrayZ[index].first <= heap.arrayZ[left].first
+        && (!has_right || heap[index].first <= heap.arrayZ[right].first))
       return;
 
-    if (!has_right || heap[left].first < heap[right].first)
+    if (!has_right || heap.arrayZ[left].first < heap.arrayZ[right].first)
     {
       swap (index, left);
       bubble_down (left);
@@ -130,10 +134,12 @@ struct hb_priority_queue_t
 
   void bubble_up (unsigned index)
   {
+    assert (index <= heap.length);
+
     if (index == 0) return;
 
     unsigned parent_index = parent (index);
-    if (heap[parent_index].first <= heap[index].first)
+    if (heap.arrayZ[parent_index].first <= heap.arrayZ[index].first)
       return;
 
     swap (index, parent_index);
diff --git a/src/test-priority-queue.cc b/src/test-priority-queue.cc
index fab63acb6..e83d72c7e 100644
--- a/src/test-priority-queue.cc
+++ b/src/test-priority-queue.cc
@@ -73,17 +73,9 @@ test_extract ()
   assert (queue.is_empty ());
 }
 
-static void
-test_extract_empty ()
-{
-  hb_priority_queue_t queue;
-  assert (queue.pop_minimum () == hb_pair (0, 0));
-}
-
 int
 main (int argc, char **argv)
 {
   test_insert ();
   test_extract ();
-  test_extract_empty ();
 }