diff --git a/test/shared_ptr_alloc2_test.cpp b/test/shared_ptr_alloc2_test.cpp
index 3eb3c46..93f9d26 100644
--- a/test/shared_ptr_alloc2_test.cpp
+++ b/test/shared_ptr_alloc2_test.cpp
@@ -41,7 +41,7 @@ public:
     typedef T const & const_reference;
     typedef T value_type;
     typedef std::size_t size_type;
-    typedef ptrdiff_t difference_type;
+    typedef std::ptrdiff_t difference_type;
 
 private: