diff --git a/include/boost/unordered/detail/hash_table.hpp b/include/boost/unordered/detail/hash_table.hpp
index f3eba852..234a9071 100644
--- a/include/boost/unordered/detail/hash_table.hpp
+++ b/include/boost/unordered/detail/hash_table.hpp
@@ -81,6 +81,7 @@ namespace boost {
         template<typename T> struct prime_list_template
         {
             static std::size_t const value[];
+            static std::ptrdiff_t const length;
         };
 
         template<typename T>
@@ -92,13 +93,16 @@ namespace boost {
             50331653ul, 100663319ul, 201326611ul, 402653189ul, 805306457ul,
             1610612741ul, 3221225473ul, 4294967291ul };
 
+        template<typename T>
+        std::ptrdiff_t const prime_list_template<T>::length = 28;
+
         typedef prime_list_template<std::size_t> prime_list;
 
         // no throw
         inline std::size_t next_prime(std::size_t n) {
             std::size_t const* const prime_list_begin = prime_list::value;
             std::size_t const* const prime_list_end = prime_list_begin +
-                sizeof(prime_list::value) / sizeof(*prime_list::value);
+                prime_list::length;
             std::size_t const* bound =
                 std::lower_bound(prime_list_begin, prime_list_end, n);
             if(bound == prime_list_end)
@@ -110,7 +114,7 @@ namespace boost {
         inline std::size_t prev_prime(std::size_t n) {
             std::size_t const* const prime_list_begin = prime_list::value;
             std::size_t const* const prime_list_end = prime_list_begin +
-                sizeof(prime_list::value) / sizeof(*prime_list::value);
+                prime_list::length;
             std::size_t const* bound =
                 std::upper_bound(prime_list_begin,prime_list_end, n);
             if(bound != prime_list_begin)