diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 3956345..eaf8f8d 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -23,7 +23,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, macOS-latest, windows-latest]
-        rust: [stable, 1.64.0]
+        rust: [stable, 1.65.0]
     steps:
     - name: Install Rust
       uses: hecrj/setup-rust-action@v1
diff --git a/CHANGELOG.md b/CHANGELOG.md
index fb3688f..b35b99e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,12 @@ file is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/), and
 this project adheres to [Semantic
 Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+### Changed
+
+- Requires Rust 1.65 or later.
+
 ## [0.6.0] - 2023-08-07
 
 ### Changed
@@ -85,6 +91,7 @@ Versioning](https://semver.org/spec/v2.0.0.html).
 - The [OPTICS](https://en.wikipedia.org/wiki/OPTICS_algorithm) clustering
   algorithm.
 
+[Unreleased]: https://github.com/petabi/petal-clustering/compare/0.6.0...main
 [0.6.0]: https://github.com/petabi/petal-clustering/compare/0.5.1...0.6.0
 [0.5.1]: https://github.com/petabi/petal-clustering/compare/0.5.0...0.5.1
 [0.5.0]: https://github.com/petabi/petal-clustering/compare/0.4.0...0.5.0
diff --git a/Cargo.toml b/Cargo.toml
index 6eeae85..3526807 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -2,7 +2,7 @@
 name = "petal-clustering"
 version = "0.6.0"
 edition = "2021"
-rust-version = "1.64"
+rust-version = "1.65"
 description = "A collection of clustering algorithms."
 readme = "README.md"
 documentation = "https://docs.rs/petal-clustering"
diff --git a/src/optics.rs b/src/optics.rs
index f51db55..1079325 100644
--- a/src/optics.rs
+++ b/src/optics.rs
@@ -91,9 +91,9 @@ where
                 if clusters.is_empty() {
                     outliers.push(id);
                 } else {
-                    let v = clusters
-                        .get_mut(&(clusters.len() - 1))
-                        .expect("cluster map crashed");
+                    let Some(v) = clusters.get_mut(&(clusters.len() - 1)) else {
+                        unreachable!("`clusters` is not empty and its key is a sequence number");
+                    };
                     v.push(id);
                 }
             } else {