diff --git a/include/cppast/cpp_entity_index.hpp b/include/cppast/cpp_entity_index.hpp
index 031b50f..8e26a8a 100644
--- a/include/cppast/cpp_entity_index.hpp
+++ b/include/cppast/cpp_entity_index.hpp
@@ -5,6 +5,7 @@
 #ifndef CPPAST_CPP_ENTITY_INDEX_HPP_INCLUDED
 #define CPPAST_CPP_ENTITY_INDEX_HPP_INCLUDED
 
+#include <cstdint>
 #include <mutex>
 #include <string>
 #include <unordered_map>
@@ -23,20 +24,21 @@ class cpp_namespace;
 /// \exclude
 namespace detail
 {
-    constexpr std::size_t fnv_basis = 14695981039346656037ull;
-    constexpr std::size_t fnv_prime = 1099511628211ull;
+    using hash_type               = std::uint_least64_t;
+    constexpr hash_type fnv_basis = 14695981039346656037ull;
+    constexpr hash_type fnv_prime = 1099511628211ull;
 
     // FNV-1a 64 bit hash
-    constexpr std::size_t id_hash(const char* str, std::size_t hash = fnv_basis)
+    constexpr hash_type id_hash(const char* str, hash_type hash = fnv_basis)
     {
-        return *str ? id_hash(str + 1, (hash ^ std::size_t(*str)) * fnv_prime) : hash;
+        return *str ? id_hash(str + 1, (hash ^ hash_type(*str)) * fnv_prime) : hash;
     }
 } // namespace detail
 
 /// A [ts::strong_typedef]() representing the unique id of a [cppast::cpp_entity]().
 ///
 /// It is comparable for equality.
-struct cpp_entity_id : type_safe::strong_typedef<cpp_entity_id, std::size_t>,
+struct cpp_entity_id : type_safe::strong_typedef<cpp_entity_id, detail::hash_type>,
                        type_safe::strong_typedef_op::equality_comparison<cpp_entity_id>
 {
     explicit cpp_entity_id(const std::string& str) : cpp_entity_id(str.c_str()) {}
@@ -118,7 +120,7 @@ private:
     {
         std::size_t operator()(const cpp_entity_id& id) const noexcept
         {
-            return static_cast<std::size_t>(id);
+            return std::size_t(static_cast<detail::hash_type>(id));
         }
     };