diff --git a/hyracks-storage-am-lsm-btree/src/main/java/edu/uci/ics/hyracks/storage/am/lsm/btree/impls/LSMBTree.java b/hyracks-storage-am-lsm-btree/src/main/java/edu/uci/ics/hyracks/storage/am/lsm/btree/impls/LSMBTree.java
index 005e712..3f88dce 100644
--- a/hyracks-storage-am-lsm-btree/src/main/java/edu/uci/ics/hyracks/storage/am/lsm/btree/impls/LSMBTree.java
+++ b/hyracks-storage-am-lsm-btree/src/main/java/edu/uci/ics/hyracks/storage/am/lsm/btree/impls/LSMBTree.java
@@ -549,7 +549,9 @@
     public void endBulkLoad(IIndexBulkLoadContext ictx) throws HyracksDataException {
         LSMTreeBulkLoadContext bulkLoadCtx = (LSMTreeBulkLoadContext) ictx;
         bulkLoadCtx.getBTree().endBulkLoad(bulkLoadCtx.getBulkLoadCtx());
-        diskBTrees.addFirst(bulkLoadCtx.getBTree());
+        synchronized (diskBTrees) {
+            diskBTrees.addFirst(bulkLoadCtx.getBTree());
+        }
     }
 
     @Override