diff --git a/tokio/src/io/async_read.rs b/tokio/src/io/async_read.rs
index d28280a51..24c1b4efb 100644
--- a/tokio/src/io/async_read.rs
+++ b/tokio/src/io/async_read.rs
@@ -123,7 +123,9 @@ pub trait AsyncRead {
                 // Convert to `&mut [u8]`
                 let b = &mut *(b as *mut [MaybeUninit<u8>] as *mut [u8]);
 
-                ready!(self.poll_read(cx, b))?
+                let n = ready!(self.poll_read(cx, b))?;
+                assert!(n <= b.len(), "Bad AsyncRead implementation, more bytes were reported as read than the buffer can hold");
+                n
             };
 
             buf.advance_mut(n);
diff --git a/tokio/tests/io_read.rs b/tokio/tests/io_read.rs
index d18615e45..4791c9a66 100644
--- a/tokio/tests/io_read.rs
+++ b/tokio/tests/io_read.rs
@@ -36,3 +36,25 @@ async fn read() {
     assert_eq!(n, 11);
     assert_eq!(buf[..], b"hello world"[..]);
 }
+
+struct BadAsyncRead;
+
+impl AsyncRead for BadAsyncRead {
+    fn poll_read(
+        self: Pin<&mut Self>,
+        _cx: &mut Context<'_>,
+        buf: &mut [u8],
+    ) -> Poll<io::Result<usize>> {
+        for b in &mut *buf {
+            *b = b'a';
+        }
+        Poll::Ready(Ok(buf.len() * 2))
+    }
+}
+
+#[tokio::test]
+#[should_panic]
+async fn read_buf_bad_async_read() {
+    let mut buf = Vec::with_capacity(10);
+    BadAsyncRead.read_buf(&mut buf).await.unwrap();
+}