diff --git a/.github/workflows/oxipng.yml b/.github/workflows/oxipng.yml
index 4c4d10af..3361fcb7 100644
--- a/.github/workflows/oxipng.yml
+++ b/.github/workflows/oxipng.yml
@@ -49,9 +49,9 @@ jobs:
           - target: i686-pc-windows-msvc
             os: windows-latest
           - target: x86_64-apple-darwin
-            os: macos-latest
+            os: macos-12
           - target: aarch64-apple-darwin
-            os: macos-latest
+            os: macos-14 # ARM64 runner
 
     env:
       CARGO_BUILD_TARGET: ${{ matrix.target }}
@@ -122,19 +122,11 @@ jobs:
           reporter: github-check
           fail_on_error: true
 
-      # There aren't good user-mode ARM64 emulators we can use on x64 macOS hosts.
-      # QEMU doesn't have any plans to add such support due to a lack of kernel
-      # syscall stability guarantees: https://gitlab.com/qemu-project/qemu/-/issues/1682
       - name: Run tests
-        if: matrix.target != 'aarch64-apple-darwin'
         run: |
           cargo nextest run --release --features sanity-checks
           cargo test --doc --release --features sanity-checks
 
-      - name: Build tests (ARM64 macOS only)
-        if: matrix.target == 'aarch64-apple-darwin'
-        run: cargo test --release --features sanity-checks --no-run
-
       - name: Build benchmarks
         run: cargo bench --no-run
 
diff --git a/src/png/scan_lines.rs b/src/png/scan_lines.rs
index 0c31dd4b..080c211e 100644
--- a/src/png/scan_lines.rs
+++ b/src/png/scan_lines.rs
@@ -24,20 +24,26 @@ impl<'a> Iterator for ScanLines<'a> {
     type Item = ScanLine<'a>;
     #[inline]
     fn next(&mut self) -> Option<Self::Item> {
-        self.iter.next().map(|(len, pass, num_pixels)| {
-            let (data, rest) = self.raw_data.split_at(len);
-            self.raw_data = rest;
-            let (&filter, data) = if self.has_filter {
-                data.split_first().unwrap()
-            } else {
-                (&0, data)
-            };
-            ScanLine {
-                filter,
-                data,
-                pass,
-                num_pixels,
-            }
+        let (len, pass, num_pixels) = self.iter.next()?;
+        debug_assert!(self.raw_data.len() >= len);
+        debug_assert!(!self.has_filter || len > 1);
+        // The data length should always be correct here but this check assures
+        // the compiler that it doesn't need to account for a potential panic
+        if self.raw_data.len() < len {
+            return None;
+        }
+        let (data, rest) = self.raw_data.split_at(len);
+        self.raw_data = rest;
+        let (&filter, data) = if self.has_filter {
+            data.split_first().unwrap()
+        } else {
+            (&0, data)
+        };
+        Some(ScanLine {
+            filter,
+            data,
+            pass,
+            num_pixels,
         })
     }
 }
diff --git a/src/reduction/palette.rs b/src/reduction/palette.rs
index aa903559..ce5459e3 100644
--- a/src/reduction/palette.rs
+++ b/src/reduction/palette.rs
@@ -14,9 +14,8 @@ pub fn reduced_palette(png: &PngImage, optimize_alpha: bool) -> Option<PngImage>
     if png.ihdr.bit_depth != BitDepth::Eight {
         return None;
     }
-    let palette = match &png.ihdr.color_type {
-        ColorType::Indexed { palette } if palette.len() > 1 => palette,
-        _ => return None,
+    let ColorType::Indexed { palette } = &png.ihdr.color_type else {
+        return None;
     };
 
     let mut used = [false; 256];
@@ -43,8 +42,9 @@ pub fn reduced_palette(png: &PngImage, optimize_alpha: bool) -> Option<PngImage>
     let data = if did_change {
         // Reassign data bytes to new indices
         png.data.iter().map(|b| byte_map[*b as usize]).collect()
-    } else if condensed.len() < palette.len() {
-        // Data is unchanged but palette will be truncated
+    } else if condensed.len() != palette.len() {
+        // Data is unchanged but palette is different size
+        // Note the new palette could potentially be larger if the original had a missing entry
         png.data.clone()
     } else {
         // Nothing has changed
@@ -183,23 +183,36 @@ pub fn sorted_palette_battiato(png: &PngImage) -> Option<PngImage> {
 
 // Find the most popular color on the image edges (the pixels neighboring the filter bytes)
 fn most_popular_edge_color(num_colors: usize, png: &PngImage) -> usize {
-    let mut counts = vec![0; num_colors];
+    let mut counts = [0u32; 256];
     for line in png.scan_lines(false) {
-        counts[line.data[0] as usize] += 1;
-        counts[line.data[line.data.len() - 1] as usize] += 1;
+        if let &[first, .., last] = line.data {
+            counts[first as usize] += 1;
+            counts[last as usize] += 1;
+        }
     }
-    counts.iter().enumerate().max_by_key(|(_, &v)| v).unwrap().0
+    counts
+        .iter()
+        .copied()
+        .take(num_colors)
+        .enumerate()
+        .max_by_key(|&(_, v)| v)
+        .unwrap_or_default()
+        .0
 }
 
 // Calculate co-occurences matrix
-fn co_occurrence_matrix(num_colors: usize, png: &PngImage) -> Vec<Vec<usize>> {
-    let mut matrix = vec![vec![0; num_colors]; num_colors];
+fn co_occurrence_matrix(num_colors: usize, png: &PngImage) -> Vec<Vec<u32>> {
+    let mut matrix = vec![vec![0u32; num_colors]; num_colors];
     let mut prev: Option<ScanLine> = None;
+    let mut prev_val = None;
     for line in png.scan_lines(false) {
         for i in 0..line.data.len() {
             let val = line.data[i] as usize;
-            if i > 0 {
-                matrix[line.data[i - 1] as usize][val] += 1;
+            if val > num_colors {
+                continue;
+            }
+            if let Some(prev_val) = prev_val.replace(val) {
+                matrix[prev_val][val] += 1;
             }
             if let Some(prev) = &prev {
                 matrix[prev.data[i] as usize][val] += 1;
@@ -211,7 +224,7 @@ fn co_occurrence_matrix(num_colors: usize, png: &PngImage) -> Vec<Vec<usize>> {
 }
 
 // Calculate edge list sorted by weight
-fn weighted_edges(matrix: &[Vec<usize>]) -> Vec<(usize, usize)> {
+fn weighted_edges(matrix: &[Vec<u32>]) -> Vec<(usize, usize)> {
     let mut edges = Vec::new();
     for i in 0..matrix.len() {
         for j in 0..i {
diff --git a/tests/files/palette_should_be_reduced_with_missing.png b/tests/files/palette_should_be_reduced_with_missing.png
new file mode 100644
index 00000000..fd6d61b8
Binary files /dev/null and b/tests/files/palette_should_be_reduced_with_missing.png differ
diff --git a/tests/reduction.rs b/tests/reduction.rs
index 221e320d..7a6292c4 100644
--- a/tests/reduction.rs
+++ b/tests/reduction.rs
@@ -1049,6 +1049,43 @@ fn palette_should_be_reduced_with_both() {
     remove_file(output).ok();
 }
 
+#[test]
+fn palette_should_be_reduced_with_missing() {
+    let input = PathBuf::from("tests/files/palette_should_be_reduced_with_missing.png");
+    let (output, opts) = get_opts(&input);
+
+    let png = PngData::new(&input, &opts).unwrap();
+
+    assert_eq!(png.raw.ihdr.color_type.png_header_code(), INDEXED);
+    assert_eq!(png.raw.ihdr.bit_depth, BitDepth::Eight);
+    if let ColorType::Indexed { palette } = &png.raw.ihdr.color_type {
+        assert_eq!(palette.len(), 2);
+    }
+
+    match oxipng::optimize(&InFile::Path(input), &output, &opts) {
+        Ok(_) => (),
+        Err(x) => panic!("{}", x),
+    };
+    let output = output.path().unwrap();
+    assert!(output.exists());
+
+    let png = match PngData::new(output, &opts) {
+        Ok(x) => x,
+        Err(x) => {
+            remove_file(output).ok();
+            panic!("{}", x)
+        }
+    };
+
+    assert_eq!(png.raw.ihdr.color_type.png_header_code(), INDEXED);
+    assert_eq!(png.raw.ihdr.bit_depth, BitDepth::Two);
+    if let ColorType::Indexed { palette } = &png.raw.ihdr.color_type {
+        assert_eq!(palette.len(), 3);
+    }
+
+    remove_file(output).ok();
+}
+
 #[test]
 fn rgba_16_reduce_alpha() {
     test_it_converts(