diff --git a/libp2p/stream_muxer/mplex/mplex.py b/libp2p/stream_muxer/mplex/mplex.py
index 00df44c..f154a16 100644
--- a/libp2p/stream_muxer/mplex/mplex.py
+++ b/libp2p/stream_muxer/mplex/mplex.py
@@ -75,7 +75,8 @@ class Mplex(IMuxedConn):
 
     async def read_buffer(self, stream_id: int) -> bytes:
         """
-        Read a message from stream_id's buffer, check raw connection for new messages
+        Read a message from stream_id's buffer, check raw connection for new messages.
+        `StreamNotFound` is raised when stream `stream_id` is not found in `Mplex`.
         :param stream_id: stream id of stream to read from
         :return: message read
         """
@@ -86,6 +87,7 @@ class Mplex(IMuxedConn):
     async def read_buffer_nonblocking(self, stream_id: int) -> Optional[bytes]:
         """
         Read a message from `stream_id`'s buffer, non-blockingly.
+        `StreamNotFound` is raised when stream `stream_id` is not found in `Mplex`.
         """
         if stream_id not in self.buffers:
             raise StreamNotFound(f"stream {stream_id} is not found")
diff --git a/libp2p/stream_muxer/mplex/mplex_stream.py b/libp2p/stream_muxer/mplex/mplex_stream.py
index 1d1d7a6..2ec23f1 100644
--- a/libp2p/stream_muxer/mplex/mplex_stream.py
+++ b/libp2p/stream_muxer/mplex/mplex_stream.py
@@ -46,14 +46,15 @@ class MplexStream(IMuxedStream):
         :param n: number of bytes to read
         :return: bytes actually read
         """
+        # TODO: Handle `StreamNotFound` raised in `self.mplex_conn.read_buffer`.
+        # TODO: Add exceptions and handle/raise them in this class.
         if n < 0 and n != -1:
-            raise ValueError(f"the number of bytes to read ``n`` must be positive or -1 to indicate read until EOF")
+            raise ValueError(
+                f"the number of bytes to read `n` must be positive or -1 to indicate read until EOF"
+            )
         # If the buffer is empty at first, blocking wait for data.
         if len(self._buf) == 0:
             self._buf.extend(await self.mplex_conn.read_buffer(self.stream_id))
-        # Sanity check: `self._buf` should never be empty here.
-        if self._buf is None or len(self._buf) == 0:
-            raise Exception("`self._buf` should never be empty here")
 
         # FIXME: If `n == -1`, we should blocking read until EOF, instead of returning when
         #   no message is available.