diff --git a/mitogen/core.py b/mitogen/core.py
index 6a1c3a64..c11d7065 100644
--- a/mitogen/core.py
+++ b/mitogen/core.py
@@ -766,14 +766,14 @@ class Stream(BasicStream):
 
         msg = Message()
         msg.router = self._router
-
         (msg.dst_id, msg.src_id, msg.auth_id,
          msg.handle, msg.reply_to, msg_len) = struct.unpack(
             self.HEADER_FMT,
             self._input_buf[0][:self.HEADER_LEN],
         )
 
-        if (self._input_buf_len - self.HEADER_LEN) < msg_len:
+        total_len = msg_len + self.HEADER_LEN
+        if self._input_buf_len < total_len:
             _vv and IOLOG.debug(
                 '%r: Input too short (want %d, got %d)',
                 self, msg_len, self._input_buf_len - self.HEADER_LEN
@@ -782,7 +782,7 @@ class Stream(BasicStream):
 
         start = self.HEADER_LEN
         prev_start = start
-        remain = msg_len + start
+        remain = total_len
         bits = []
         while remain:
             buf = self._input_buf.popleft()
@@ -794,7 +794,7 @@ class Stream(BasicStream):
 
         msg.data = ''.join(bits)
         self._input_buf.appendleft(buf[prev_start+len(bit):])
-        self._input_buf_len -= self.HEADER_LEN + msg_len
+        self._input_buf_len -= total_len
         self._router._async_route(msg, self)
         return True