diff --git a/include/boost/detail/sp_counted_base_pt.hpp b/include/boost/detail/sp_counted_base_pt.hpp
index 5fb4716..191064f 100644
--- a/include/boost/detail/sp_counted_base_pt.hpp
+++ b/include/boost/detail/sp_counted_base_pt.hpp
@@ -37,7 +37,7 @@ private:
     long use_count_;        // #shared
     long weak_count_;       // #weak + (#shared != 0)
 
-    pthread_mutex_t m_;
+    mutable pthread_mutex_t m_;
 
 public: