diff --git a/include/boost/gil/extension/io/detail/scanline_read_iterator.hpp b/include/boost/gil/extension/io/detail/scanline_read_iterator.hpp
index 94103f5c3..e83e8543f 100644
--- a/include/boost/gil/extension/io/detail/scanline_read_iterator.hpp
+++ b/include/boost/gil/extension/io/detail/scanline_read_iterator.hpp
@@ -35,6 +35,7 @@ public:
     typedef byte_t* value_type;
     typedef value_type const* pointer;
     typedef value_type const& reference;
+    typedef int difference_type;
     
     /// todo
     //typedef ptrdiff_t difference_type;