diff --git a/common/commonspace/diffservice/diffsyncer.go b/common/commonspace/diffservice/diffsyncer.go
index 49947c0a..d5cb1c3a 100644
--- a/common/commonspace/diffservice/diffsyncer.go
+++ b/common/commonspace/diffservice/diffsyncer.go
@@ -129,7 +129,9 @@ func (d *diffSyncer) pingTreesInCache(ctx context.Context, trees []string) {
 		// it may be already there (i.e. loaded)
 		// and build func will not be called, thus we won't sync the tree
 		// therefore we just do it manually
+		syncTree.Lock()
 		syncTree.Ping()
+		syncTree.Unlock()
 	}
 }