diff --git a/ompi/debuggers/ompi_msgq_dll.c b/ompi/debuggers/ompi_msgq_dll.c
index 134e6f35dd..8ccdb8d35c 100644
--- a/ompi/debuggers/ompi_msgq_dll.c
+++ b/ompi/debuggers/ompi_msgq_dll.c
@@ -149,6 +149,9 @@ opal_condition_t ompi_request_cond;
 size_t ompi_request_waiting;
 opal_mutex_t ompi_request_lock;
 opal_mutex_t opal_event_lock;
+#if OMPI_HAVE_THREAD_SUPPORT
+volatile
+#endif
 uint32_t opal_progress_recursion_depth_counter;
 int opal_progress_spin_count;
 volatile int32_t opal_progress_thread_count;