diff --git a/package.json b/package.json
index 7c76c05..dfba269 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "serializable-bptree",
-  "version": "4.0.3",
+  "version": "4.0.4",
   "description": "Store the B+tree flexibly, not only in-memory.",
   "types": "./dist/types/index.d.ts",
   "main": "./dist/cjs/index.cjs",
diff --git a/src/BPTreeAsync.ts b/src/BPTreeAsync.ts
index 919e2ea..643cd57 100644
--- a/src/BPTreeAsync.ts
+++ b/src/BPTreeAsync.ts
@@ -173,8 +173,11 @@ export class BPTreeAsync<K, V> extends BPTree<K, V> {
       (node.keys.length < Math.ceil(this.order/2) && !node.leaf) ||
       (node.values.length < Math.ceil((this.order-1)/2) && node.leaf)
     ) {
+      if (node.parent === null) {
+        return
+      }
       let isPredecessor = false
-      let parentNode = await this.getNode(node.parent!) as BPTreeInternalNode<K, V>
+      let parentNode = await this.getNode(node.parent) as BPTreeInternalNode<K, V>
       let prevNode: BPTreeInternalNode<K, V>|null = null
       let nextNode: BPTreeInternalNode<K, V>|null = null
       let prevValue: V|null = null
diff --git a/src/BPTreeSync.ts b/src/BPTreeSync.ts
index 0345447..b07522b 100644
--- a/src/BPTreeSync.ts
+++ b/src/BPTreeSync.ts
@@ -173,8 +173,11 @@ export class BPTreeSync<K, V> extends BPTree<K, V> {
       (node.keys.length < Math.ceil(this.order/2) && !node.leaf) ||
       (node.values.length < Math.ceil((this.order-1)/2) && node.leaf)
     ) {
+      if (node.parent === null) {
+        return
+      }
       let isPredecessor = false
-      let parentNode = this.getNode(node.parent!) as BPTreeInternalNode<K, V>
+      let parentNode = this.getNode(node.parent) as BPTreeInternalNode<K, V>
       let prevNode: BPTreeInternalNode<K, V>|null = null
       let nextNode: BPTreeInternalNode<K, V>|null = null
       let prevValue: V|null = null