diff --git a/include/gsl/span b/include/gsl/span
index 4966961..5900da5 100644
--- a/include/gsl/span
+++ b/include/gsl/span
@@ -128,7 +128,7 @@ namespace details
     public:
         using iterator_category = std::random_access_iterator_tag;
         using value_type = std::remove_cv_t<type>;
-        using difference_type = ptrdiff_t;
+        using difference_type = std::ptrdiff_t;
         using pointer = std::add_pointer_t<type>;
         using reference = std::add_lvalue_reference_t<type>;