diff --git a/src/lib.rs b/src/lib.rs
index b9ba393f..096cf459 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -215,7 +215,7 @@ pub fn optimize(filepath: &Path, opts: &Options) -> Result<(), String> {
             let mut buffer = BufWriter::new(io::stdout());
             match buffer.write_all(&output_data) {
                 Ok(_) => (),
-                Err(_) => return Err(format!("Unable to write to stdout")),
+                Err(_) => return Err("Unable to write to stdout".to_owned()),
             }
         } else {
             let out_file = match File::create(&opts.out_file) {
diff --git a/src/png.rs b/src/png.rs
index 8f763125..532790a1 100644
--- a/src/png.rs
+++ b/src/png.rs
@@ -97,7 +97,7 @@ struct ScanLines<'a> {
 }
 
 impl<'a> ScanLines<'a> {
-    fn len(&mut self) -> usize {
+    fn len(&self) -> usize {
         self.png.raw_data.len()
     }
 }
@@ -759,6 +759,7 @@ fn reduce_rgba_to_rgb(png: &PngData) -> Option<Vec<u8>> {
                 if *byte != 255 {
                     return None;
                 }
+            } else {
                 reduced.push(*byte);
             }
         }