diff --git a/typeidx.cpp b/typeidx.cpp
index 03b0fd59..6bb861f1 100644
--- a/typeidx.cpp
+++ b/typeidx.cpp
@@ -16,11 +16,12 @@
 
 #include "typeidx.hpp"
 
+#include <atomic>
 
 ///////////////////////////////////////////////////////////////////////////////
-std::uintptr_t
+int
 util::detail::typeidx_next ()
 {
-    static std::uintptr_t counter = 0;
+    static std::atomic<int> counter = 0;
     return counter++;
 }
diff --git a/typeidx.hpp b/typeidx.hpp
index 31a5c849..21a162f1 100644
--- a/typeidx.hpp
+++ b/typeidx.hpp
@@ -20,7 +20,7 @@
 #include <cstdint>
 
 namespace util {
-    namespace detail { std::uintptr_t typeidx_next (void); }
+    namespace detail { int typeidx_next (void); }
 
     /// return a globally unique runtime ID for a given type.
     ///
@@ -29,13 +29,13 @@ namespace util {
     ///
     /// the identifier is constructed at runtime and is not guaranteed to be
     /// stable across executions (particularly in the case of threads and
-    /// other non-determinism).
+    /// other non-determinism). however it _is_ threadsafe to call this.
     ///
     /// the range of identifiers is _probably_ contiguous starting from zero.
     /// this should not be relied upon for correctness, but may be used for
     /// performance concerns.
     template <typename T>
-    std::uintptr_t
+    int
     typeidx (void)
     {
         static auto id = detail::typeidx_next ();