diff --git a/src/mca/ptl/base/ptl_base_sendreq.h b/src/mca/ptl/base/ptl_base_sendreq.h
index bcbd76a6a4..19208b9703 100644
--- a/src/mca/ptl/base/ptl_base_sendreq.h
+++ b/src/mca/ptl/base/ptl_base_sendreq.h
@@ -58,7 +58,7 @@ static inline void mca_ptl_base_send_request_offset(
     mca_ptl_base_send_request_t* request,
     size_t offset)
 {
-    OMPI_THREAD_ADD32((size_t*)(&request->req_offset), offset);
+    OMPI_THREAD_ADD_SIZE_T((&request->req_offset), offset);
 }
 
 
diff --git a/src/threads/mutex.h b/src/threads/mutex.h
index 818b5c6021..4e99c419df 100644
--- a/src/threads/mutex.h
+++ b/src/threads/mutex.h
@@ -258,11 +258,25 @@ static inline bool ompi_set_using_threads(bool have)
 #if OMPI_HAVE_THREAD_SUPPORT
 #define OMPI_THREAD_ADD64(x,y) \
    ((OMPI_HAVE_THREAD_SUPPORT && ompi_using_threads()) ? \
-    ompi_atomic_add_32(x,y) : (*x += y))
+    ompi_atomic_add_64(x,y) : (*x += y))
 #else
 #define OMPI_THREAD_ADD64(x,y) (*x += y)
 #endif
 
+#if OMPI_HAVE_THREAD_SUPPORT
+#if OMPI_SIZEOF_SIZE_T == 4
+#define OMPI_THREAD_ADD_SIZE_T(x,y) \
+    ((OMPI_HAVE_THREAD_SUPPORT && ompi_using_threads()) ?       \
+     ompi_atomic_add_32(x,y) : (*x += y))
+#elif OMPI_SIZEOF_SIZE_T == 8
+#define OMPI_THREAD_ADD_SIZE_T(x,y) \
+    ((OMPI_HAVE_THREAD_SUPPORT && ompi_using_threads()) ?       \
+     ompi_atomic_add_64(x,y) : (*x += y))
+#endif
+#else
+#define OMPI_THREAD_ADD_SIZE_T(x,y) (*x += y)
+#endif
+
 
 /**
  * Always locks a mutex (never compile- or run-time removed)