diff --git a/src/snmalloc/ds/flaglock.h b/src/snmalloc/ds/flaglock.h
index fd07d3cfe..4a539e636 100644
--- a/src/snmalloc/ds/flaglock.h
+++ b/src/snmalloc/ds/flaglock.h
@@ -15,6 +15,8 @@ namespace snmalloc
    */
   struct DebugFlagWord
   {
+    using ThreadIdentity = DefaultPal::ThreadIdentity;
+
     /**
      * @brief flag
      * The underlying atomic field.
@@ -33,7 +35,7 @@ namespace snmalloc
      */
     void set_owner()
     {
-      SNMALLOC_ASSERT(0 == owner);
+      SNMALLOC_ASSERT(ThreadIdentity() == owner);
       owner = get_thread_identity();
     }
 
@@ -44,7 +46,7 @@ namespace snmalloc
     void clear_owner()
     {
       SNMALLOC_ASSERT(get_thread_identity() == owner);
-      owner = 0;
+      owner = ThreadIdentity();
     }
 
     /**
@@ -57,18 +59,17 @@ namespace snmalloc
     }
 
   private:
-    using ThreadIdentity = size_t;
     /**
      * @brief owner
      * We use the Pal to provide the ThreadIdentity.
      */
-    std::atomic<ThreadIdentity> owner = 0;
+    std::atomic<ThreadIdentity> owner = ThreadIdentity();
 
     /**
      * @brief get_thread_identity
      * @return The identity of current thread.
      */
-    static size_t get_thread_identity()
+    static ThreadIdentity get_thread_identity()
     {
       return DefaultPal::get_tid();
     }
diff --git a/src/snmalloc/pal/pal.h b/src/snmalloc/pal/pal.h
index b48e2591c..31b846d67 100644
--- a/src/snmalloc/pal/pal.h
+++ b/src/snmalloc/pal/pal.h
@@ -177,7 +177,8 @@ namespace snmalloc
   inline void message(Args... args)
   {
     MessageBuilder<BufferSize> msg{std::forward<Args>(args)...};
-    MessageBuilder<BufferSize> msg_tid{"{}: {}", DefaultPal::get_tid(), msg.get_message()};
+    MessageBuilder<BufferSize> msg_tid{
+      "{}: {}", DefaultPal::get_tid(), msg.get_message()};
     DefaultPal::message(msg_tid.get_message());
   }
 } // namespace snmalloc
diff --git a/src/snmalloc/pal/pal_concept.h b/src/snmalloc/pal/pal_concept.h
index f16455093..789a00b6d 100644
--- a/src/snmalloc/pal/pal_concept.h
+++ b/src/snmalloc/pal/pal_concept.h
@@ -78,7 +78,8 @@ namespace snmalloc
 
   /**
    * The Pal must provide a thread id for debugging.  It should not return
-   * 0, as that is used as not an tid in some places.
+   * the default value of ThreadIdentity, as that is used as not an tid in some
+   * places.
    */
   template<typename PAL>
   concept ConceptPAL_tid = requires()
@@ -86,7 +87,7 @@ namespace snmalloc
     {
       PAL::get_tid()
     }
-    noexcept->ConceptSame<size_t>;
+    noexcept->ConceptSame<typename PAL::ThreadIdentity>;
   };
 
   /**
@@ -151,7 +152,7 @@ namespace snmalloc
   template<typename PAL>
   concept ConceptPAL = ConceptPAL_static_features<PAL>&&
                          ConceptPAL_static_sizes<PAL>&& ConceptPAL_error<PAL>&&
-                           ConceptPAL_memops<PAL> &&  ConceptPAL_tid<PAL> &&
+                           ConceptPAL_memops<PAL>&& ConceptPAL_tid<PAL> &&
     (!pal_supports<Entropy, PAL> ||
      ConceptPAL_get_entropy64<
        PAL>)&&(!pal_supports<LowMemoryNotification, PAL> ||
diff --git a/src/snmalloc/pal/pal_tid_default.h b/src/snmalloc/pal/pal_tid_default.h
index 8ec3d93d5..678af98b0 100644
--- a/src/snmalloc/pal/pal_tid_default.h
+++ b/src/snmalloc/pal/pal_tid_default.h
@@ -7,13 +7,15 @@ namespace snmalloc
   class PalTidDefault
   {
   public:
+    using ThreadIdentity = size_t;
+
     /**
      * @brief Get the an id for the current thread.
-     * 
-     * @return the thread id, this should never be zero.
-     * Callers can assume it is non-zero.
+     *
+     * @return the thread id, this should never be the default of
+     * ThreadIdentity. Callers can assume it is a non-default value.
      */
-    static inline size_t get_tid() noexcept
+    static inline ThreadIdentity get_tid() noexcept
     {
       static thread_local size_t tid{0};
       static std::atomic<size_t> tid_source{0};