diff --git a/test_util/src/lsp.rs b/test_util/src/lsp.rs
index 87602e3fe0..6cc4576d8a 100644
--- a/test_util/src/lsp.rs
+++ b/test_util/src/lsp.rs
@@ -337,8 +337,20 @@ mod tests {
 
   #[test]
   fn test_read_message() {
-    let msg = b"content-length: 11\r\n\r\nhello world";
-    let mut reader = std::io::Cursor::new(msg);
-    assert_eq!(read_message(&mut reader).unwrap(), b"hello world");
+    let msg1 = b"content-length: 11\r\n\r\nhello world";
+    let mut reader1 = std::io::Cursor::new(msg1);
+    assert_eq!(read_message(&mut reader1).unwrap(), b"hello world");
+
+    let msg2 = b"content-length: 5\r\n\r\nhello world";
+    let mut reader2 = std::io::Cursor::new(msg2);
+    assert_eq!(read_message(&mut reader2).unwrap(), b"hello");
+  }
+
+  #[test]
+  #[should_panic(expected = "failed to fill whole buffer")]
+  fn test_invalid_read_message() {
+    let msg1 = b"content-length: 12\r\n\r\nhello world";
+    let mut reader1 = std::io::Cursor::new(msg1);
+    read_message(&mut reader1).unwrap();
   }
 }