diff --git a/libcxx/include/__algorithm/partial_sort.h b/libcxx/include/__algorithm/partial_sort.h
--- a/libcxx/include/__algorithm/partial_sort.h
+++ b/libcxx/include/__algorithm/partial_sort.h
@@ -33,6 +33,8 @@
 __partial_sort(_RandomAccessIterator __first, _RandomAccessIterator __middle, _RandomAccessIterator __last,
              _Compare __comp)
 {
+    if (__first == __middle)
+        return;
     _VSTD::__make_heap<_Compare>(__first, __middle, __comp);
     typename iterator_traits<_RandomAccessIterator>::difference_type __len = __middle - __first;
     for (_RandomAccessIterator __i = __middle; __i != __last; ++__i)