diff --git a/mp4/codec.py b/mp4/codec.py
index d9c7df5..e4d228f 100644
--- a/mp4/codec.py
+++ b/mp4/codec.py
@@ -1,10 +1,5 @@
 # RozK
 
-class NullCodec:
-    @property
-    def name(self):
-        return b"<None>"
-
 class Codec:
     __slots__ = '_ref'
 
diff --git a/mp4/decoder.py b/mp4/decoder.py
index 52da118..fef841f 100644
--- a/mp4/decoder.py
+++ b/mp4/decoder.py
@@ -33,7 +33,7 @@ class Decoder:
                 break
             elif errcode < 0:
                 errstring = libav.strerror(errcode)
-                raise Exception(f"Failed to receive frame: {errstring} {errcode} {libav.AVERROR_EAGAIN}")
+                raise Exception(f"Failed to receive frame: {errstring}")
             frames.append(frame)
         return frames
 
diff --git a/mp4/demuxer.py b/mp4/demuxer.py
index fbb9c9c..47f95ba 100644
--- a/mp4/demuxer.py
+++ b/mp4/demuxer.py
@@ -2,7 +2,7 @@
 
 from . import libav
 from .codec import Codec
-from .stream import NullStream, Stream
+from .stream import Stream
 from .packet import Packet
 
 class Demuxer:
@@ -20,7 +20,11 @@ class Demuxer:
             libav.format_close_input(self._context)
             raise Exception("Failed to find stream info")
         self.video_stream = self._find_stream(libav.AVMEDIA_TYPE_VIDEO)
+        if self.video_stream is None:
+            raise Exception("Failed to find a video stream")
         self.audio_stream = self._find_stream(libav.AVMEDIA_TYPE_AUDIO)
+        if self.audio_stream is None:
+            raise Exception("Failed to find an audio stream")
 
     @property
     def nb_streams(self):
@@ -31,7 +35,7 @@ class Demuxer:
     def _find_stream(self, type):
         index, codec_ref = libav.format_find_best_stream(self._context, type)
         if index < 0 or not codec_ref:
-            return NullStream()
+            return None
         parameters = self._context.contents.streams[index].contents.codecpar
         return Stream(index, Codec(codec_ref), parameters)
 
diff --git a/mp4/stream.py b/mp4/stream.py
index 0c7dace..69cbf40 100644
--- a/mp4/stream.py
+++ b/mp4/stream.py
@@ -1,23 +1,5 @@
 # RozK
 
-from .codec import NullCodec
-
-class NullStream:
-    @property
-    def index(self):
-        return -1
-
-    @property
-    def codec(self):
-        return NullCodec()
-
-    @property
-    def parameters(self):
-        return None
-
-    def contains(self, packet):
-        return False
-
 class Stream:
     __slots__ = 'index', 'codec', 'parameters'