diff --git a/implementations/rust/preserves/src/value/iolist.rs b/implementations/rust/preserves/src/value/iolist.rs
index 9061d8e..2f31226 100644
--- a/implementations/rust/preserves/src/value/iolist.rs
+++ b/implementations/rust/preserves/src/value/iolist.rs
@@ -36,7 +36,7 @@ impl IOList {
     pub fn write_all(&mut self, cow: Cow<'_, [u8]>) {
         self.len += cow.len();
         match cow {
-            Cow::Borrowed(bs) => self.chunks.push(bs.to_owned()),
+            Cow::Borrowed(bs) => self.rightmost_chunk().extend_from_slice(bs),
             Cow::Owned(bs) => self.chunks.push(bs),
         }
     }