diff --git a/src/lib.rs b/src/lib.rs
index 52d8b26..cd561d2 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -2,6 +2,8 @@
 extern crate alloc;
 
 pub mod path;
+#[cfg(test)]
+pub mod path_test;
 pub mod subtree;
 
 #[cfg(feature = "std")]
diff --git a/src/path.rs b/src/path.rs
index e7e6cf8..4456e88 100644
--- a/src/path.rs
+++ b/src/path.rs
@@ -118,8 +118,11 @@ impl<T: BitLength + AsRef<[u8]>> PathUtils for T {
         Direction::Left
     }
 
+    /// Returns the first differing bit of `self`, from `start` bit of `self`(inclusive).
+    /// If all bits are the same, returns None instead.
     fn split_point<S: BitLength + PathUtils>(&self, start: usize, b: S) -> Option<usize> {
-        let max_bit_len = core::cmp::min(self.bit_len(), b.bit_len());
+        assert!(self.bit_len() >= start);
+        let max_bit_len = core::cmp::min(self.bit_len() - start, b.bit_len());
         let (src_start_byte, src_start_bit, seg_end_byte) =
             (start / 8, start % 8, (max_bit_len + 7) / 8);
         let mut count = 0;
diff --git a/src/path_test.rs b/src/path_test.rs
new file mode 100644
index 0000000..1286f91
--- /dev/null
+++ b/src/path_test.rs
@@ -0,0 +1,19 @@
+use crate::path::{BitLength, Path, PathSegment};
+use crate::ZERO_HASH;
+
+#[test]
+fn path_bit_len() {
+    let current_key = Path(ZERO_HASH);
+    for depth in 0..256 {
+        for point in depth..256 {
+            let seg = PathSegment::from_path(current_key, depth, point);
+            assert_eq!(
+                seg.bit_len(),
+                point - depth,
+                "depth:{} point:{}",
+                depth,
+                point
+            );
+        }
+    }
+}
diff --git a/src/tx.rs b/src/tx.rs
index 6bcee9b..457b0b3 100644
--- a/src/tx.rs
+++ b/src/tx.rs
@@ -361,7 +361,7 @@ impl<'db, H: NodeHasher> WriteTransaction<'db, H> {
         let point = current_key.split_point(0, key).unwrap();
         let prefix = PathSegment::from_path(current_key, depth, point);
 
-        let depth = depth + prefix.bit_len();
+        let depth = point;
         let node_direction = key.direction(depth);
         let current_node = Node::from_leaf(current_key, current_value);
         let node = Node::from_leaf(key, value);