diff --git a/src/pty.rs b/src/pty.rs
index 3142694..1997e33 100644
--- a/src/pty.rs
+++ b/src/pty.rs
@@ -283,7 +283,6 @@ fn poll_read(
             std::task::Poll::Ready(guard) => guard,
             std::task::Poll::Pending => return std::task::Poll::Pending,
         }?;
-        let prev_filled = buf.filled().len();
         // SAFETY: we only pass b to read_buf, which never uninitializes any
         // part of the buffer it is given
         let b = unsafe { buf.unfilled_mut() };
@@ -298,7 +297,7 @@ fn poll_read(
                 // initialized previously, or the call to read_buf did), and
                 // assume_init will ignore any attempts to shrink the
                 // initialized space, so this call is always safe.
-                unsafe { buf.assume_init(prev_filled + bytes) };
+                unsafe { buf.assume_init(bytes) };
                 buf.advance(bytes);
                 return std::task::Poll::Ready(Ok(()));
             }
diff --git a/tests/basic.rs b/tests/basic.rs
index 33f5d7a..637b471 100644
--- a/tests/basic.rs
+++ b/tests/basic.rs
@@ -68,3 +68,56 @@ async fn test_yes_async() {
 
     child.kill().await.unwrap()
 }
+
+#[cfg(feature = "async")]
+#[tokio::test]
+async fn test_buffer_initialization() {
+    use std::pin::Pin;
+    use std::task::{Context, Poll};
+    use tokio::io::AsyncRead;
+
+    let (mut pty, pts) = pty_process::open().unwrap();
+    pty.resize(pty_process::Size::new(24, 80)).unwrap();
+    let msg = "hello world";
+    let mut child = pty_process::Command::new("echo")
+        .arg(msg)
+        .spawn(pts)
+        .unwrap();
+
+    let mut buf = [std::mem::MaybeUninit::uninit(); 64];
+
+    let mut read_buf = tokio::io::ReadBuf::uninit(&mut buf);
+
+    let waker = futures::task::noop_waker();
+    let mut cx = Context::from_waker(&waker);
+
+    // first read: the whole output will fit into the buffer
+    loop {
+        match Pin::new(&mut pty).poll_read(&mut cx, &mut read_buf) {
+            Poll::Ready(Ok(())) => break,
+            Poll::Pending => {
+                tokio::time::sleep(std::time::Duration::from_millis(10))
+                    .await;
+            }
+            Poll::Ready(Err(e)) => panic!("Read failed: {}", e),
+        }
+    }
+    assert_eq!(read_buf.filled().len(), msg.len() + 2); // msg + "\r\n"
+    assert_eq!(read_buf.initialized().len(), read_buf.filled().len());
+
+    // second read: there will be no new data read
+    loop {
+        match Pin::new(&mut pty).poll_read(&mut cx, &mut read_buf) {
+            Poll::Ready(Ok(())) => break,
+            Poll::Pending => {
+                tokio::time::sleep(std::time::Duration::from_millis(10))
+                    .await;
+            }
+            Poll::Ready(Err(e)) => panic!("Read failed: {}", e),
+        }
+    }
+    assert_eq!(read_buf.filled().len(), msg.len() + 2);
+    assert_eq!(read_buf.initialized().len(), read_buf.filled().len());
+
+    child.wait().await.unwrap();
+}