# must fail: head_proof_ok_O3
# the left subtree is half the length, rounded down to a chunk (head_proof::left_len)
diff --git a/wai/wai-rs/src/http_binding/head_proof.rs b/wai/wai-rs/src/http_binding/head_proof.rs
index 40b6f1fa..ea665c22 100644
--- a/wai/wai-rs/src/http_binding/head_proof.rs
+++ b/wai/wai-rs/src/http_binding/head_proof.rs
@@ -35,7 +35,7 @@ pub struct HeadProof {
 /// leaves at least one byte to the right.
 fn left_len(n: u64) -> u64 {
     let q = (n - 1) / CHUNK;
-    CHUNK << (63 - q.leading_zeros())
+    ((n / 2) / CHUNK).max(1) * CHUNK
 }
 
 /// The root of a tree from its two children: BLAKE3's parent node with the ROOT flag.