diff --git a/include/concept.h b/include/concept.h
index 6eb123f..2145e9f 100644
--- a/include/concept.h
+++ b/include/concept.h
@@ -34,7 +34,7 @@ concept explicit_convertible_to =
 &&   std::constructible_from <_To, _From>;
 
 template <typename _Tp>
-concept has_length = requires { _Tp::_Bit_Len; };
+concept has_length = requires { { +_Tp::_Bit_Len } -> std::same_as <std::size_t>; };
 
 template <typename _Tp>
 concept bit_type =  has_length <_Tp> && explicit_convertible_to <_Tp, max_size_t>;
diff --git a/include/register.h b/include/register.h
index f81849a..67220dd 100644
--- a/include/register.h
+++ b/include/register.h
@@ -26,7 +26,7 @@ struct Register {
     }
 
   public:
-    static constexpr bool _Bit_Len = _Len;
+    static constexpr std::size_t _Bit_Len = _Len;
 
     explicit Register() : _M_old(), _M_new(), _M_assigned() {}
 
diff --git a/include/wire.h b/include/wire.h
index 77bcb31..5c1bf7e 100644
--- a/include/wire.h
+++ b/include/wire.h
@@ -73,7 +73,7 @@ struct Wire {
     }
 
   public:
-    static constexpr bool _Bit_Len = _Len;
+    static constexpr std::size_t _Bit_Len = _Len;
 
     explicit Wire() :
         _M_func(new details::EmptyWire),