diff --git a/server/core/internal/mlist.h b/server/core/internal/mlist.h
index 949d8aacc..48cfafec5 100644
--- a/server/core/internal/mlist.h
+++ b/server/core/internal/mlist.h
@@ -32,7 +32,7 @@ typedef struct mlist_st
     bool           mlist_deleted;
     size_t         mlist_nodecount;
     size_t         mlist_nodecount_max; /**< size limit. 0 == no limit */
-    size_t         mlist_versno;
+    volatile size_t mlist_versno;
     bool           mlist_flat;
     mlist_node_t*  mlist_first;
     mlist_node_t*  mlist_last;