diff --git a/core/src/main/java/overflowdb/ReferenceManager.java b/core/src/main/java/overflowdb/ReferenceManager.java
index bacb11fe..1bfed5ae 100644
--- a/core/src/main/java/overflowdb/ReferenceManager.java
+++ b/core/src/main/java/overflowdb/ReferenceManager.java
@@ -152,8 +152,10 @@ private void safelyClearReferences(final List<NodeRef> refsToClear) {
    * useful when saving the graph
    */
   public void clearAllReferences() {
-    nodesWriter.writeAndClearBatched(clearableRefs.spliterator(), clearableRefs.size());
-    logger.debug("cleared all clearable references");
+    synchronized (clearableRefs){
+      nodesWriter.writeAndClearBatched(clearableRefs.spliterator(), clearableRefs.size());
+      logger.debug("cleared all clearable references");
+    }
   }
 
   @Override