diff --git a/include/boost/utility/string_ref.hpp b/include/boost/utility/string_ref.hpp
index 706967d..6e8ecc5 100644
--- a/include/boost/utility/string_ref.hpp
+++ b/include/boost/utility/string_ref.hpp
@@ -134,7 +134,7 @@ namespace boost {
         // element access
         BOOST_CONSTEXPR const charT& operator[](size_type pos) const { return ptr_[pos]; }
 
-        const charT& at(size_t pos) const {
+        const charT& at(size_type pos) const {
             if ( pos >= len_ )
                 BOOST_THROW_EXCEPTION( std::out_of_range ( "boost::string_ref::at" ) );
             return ptr_[pos];
diff --git a/include/boost/utility/string_view.hpp b/include/boost/utility/string_view.hpp
index 2db87f1..30837e4 100644
--- a/include/boost/utility/string_view.hpp
+++ b/include/boost/utility/string_view.hpp
@@ -128,7 +128,7 @@ namespace boost {
         // element access
         BOOST_CONSTEXPR const_reference operator[](size_type pos) const BOOST_NOEXCEPT { return ptr_[pos]; }
 
-        BOOST_CONSTEXPR const_reference at(size_t pos) const {
+        BOOST_CONSTEXPR const_reference at(size_type pos) const {
             return pos >= len_ ? BOOST_THROW_EXCEPTION(std::out_of_range("boost::string_view::at")), ptr_[0] : ptr_[pos];
             }