diff --git a/mini-lsm-starter/src/mvcc/watermark.rs b/mini-lsm-starter/src/mvcc/watermark.rs
index 56fb1cd..af56e63 100644
--- a/mini-lsm-starter/src/mvcc/watermark.rs
+++ b/mini-lsm-starter/src/mvcc/watermark.rs
@@ -21,6 +21,12 @@ pub struct Watermark {
     readers: BTreeMap<u64, usize>,
 }
 
+impl Default for Watermark {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 impl Watermark {
     pub fn new() -> Self {
         Self {