diff --git a/cursor.js b/cursor.js
index 84eaf5e..d251ae0 100644
--- a/cursor.js
+++ b/cursor.js
@@ -90,16 +90,14 @@ export class Cursor {
 
         if (key === "ArrowUp" && currentIndex > 0) {
             const prevSibling = siblings[currentIndex - 1]
-            const length = prevSibling.textContent.length
-            const index = length < this.insertIndex ? length : this.insertIndex
+            const index = Math.max(0, Math.min(prevSibling.textContent.length, this.insertIndex))
             this.setTarget(prevSibling, index)  // 光标在上一个兄弟元素的起始位置
             return
         }
 
         if (key === "ArrowDown" && currentIndex >= 0 && currentIndex < siblings.length - 1) {
             const nextSibling = siblings[currentIndex + 1]
-            const length = nextSibling.textContent.length
-            const index = length < this.insertIndex ? length : this.insertIndex
+            const index = Math.max(0, Math.min(nextSibling.textContent.length, this.insertIndex))
             this.setTarget(nextSibling, index)  // 光标在下一个兄弟元素的起始位置
             return
         }