diff --git a/src/spdl/io/_core.py b/src/spdl/io/_core.py
index cb6faa546..e91c44126 100644
--- a/src/spdl/io/_core.py
+++ b/src/spdl/io/_core.py
@@ -90,6 +90,7 @@ def log_api_usage_once(_: str) -> None:
 
     CUDABuffer = _libspdl_cuda.CUDABuffer
     CUDAConfig = _libspdl_cuda.CUDAConfig
+    _NvDecDecoder = _libspdl_cuda.NvDecDecoder
 
     SourceType = str | Path | bytes | UintArray | Tensor
 
@@ -723,7 +724,17 @@ def decode_packets_nvdec(
     # long_name attribute
     #
     #  "QuickTime / MOV", "FLV (Flash Video)", "Matroska / WebM"
-    match packets.codec.name:
+    if (codec := packets.codec) is None:
+        raise ValueError(
+            "The packets must have codec. "
+            "The packets object does not have codec when streaming demuxing. "
+            "The `decode_packets_nvdec` function is for one-off decoding. "
+            "If you want to use NVDEC with streaming demuxing, instantiate "
+            "NvDecDecoder object manually."
+        )
+
+    assert codec is not None  # for type-checker
+    match codec.name:
         case "h264":
             packets = apply_bsf(packets, "h264_mp4toannexb")
         case "hevc":
@@ -878,10 +889,10 @@ class NvDecDecoder:
           buffer = spdl.io.nv12_to_rgb(buffer)
     """
 
-    def __init__(self, decoder) -> None:
+    def __init__(self, decoder: "_NvDecDecoder") -> None:
         log_api_usage_once("spdl.io.NvDecDecoder")
 
-        self._decoder = decoder
+        self._decoder: "_NvDecDecoder" = decoder
 
     def init(
         self,
@@ -967,13 +978,13 @@ def flush(self) -> "list[CUDABuffer]":
 _THREAD_LOCAL = threading.local()
 
 
-def _get_decoder():
+def _get_decoder() -> "_NvDecDecoder":
     if not hasattr(_THREAD_LOCAL, "_decoder"):
-        _THREAD_LOCAL._decoder = _libspdl_cuda._nvdec_decoder()
+        _THREAD_LOCAL._decoder = _libspdl_cuda._nvdec_decoder()  # pyre-ignore[16]
     return _THREAD_LOCAL._decoder
 
 
-def nvdec_decoder(use_cache: bool = True) -> NvDecDecoder:
+def nvdec_decoder(use_cache: bool = True) -> "NvDecDecoder":
     """Instantiate an :py:class:`NvDecDecoder` object.
 
     Args:
@@ -1030,10 +1041,12 @@ def convert_frames(
             stacklevel=2,
         )
         kwargs.pop("pin_memory")
-    return _libspdl.convert_frames(frames, storage=storage, **kwargs)
+    return _libspdl.convert_frames(frames, storage=storage, **kwargs)  # pyre-ignore[6]
 
 
-def convert_array(vals, storage: "CPUStorage | None" = None) -> "CPUBuffer":
+def convert_array(
+    vals: "UintArray", storage: "CPUStorage | None" = None
+) -> "CPUBuffer":
     """Convert the given array to buffer.
 
     This function is intended to be used when sending class labels (which is
@@ -1051,7 +1064,7 @@ def convert_array(vals, storage: "CPUStorage | None" = None) -> "CPUBuffer":
 
 
 def create_reference_audio_frame(
-    array, sample_fmt: str, sample_rate: int, pts: int
+    array: "UintArray", sample_fmt: str, sample_rate: int, pts: int
 ) -> "AudioFrames":
     """Create an AudioFrame object which refers to the given array/tensor.
 
@@ -1101,7 +1114,7 @@ def create_reference_audio_frame(
 
 
 def create_reference_video_frame(
-    array, pix_fmt: str, frame_rate: tuple[int, int], pts: int
+    array: "UintArray", pix_fmt: str, frame_rate: tuple[int, int], pts: int
 ) -> "VideoFrames":
     """Create an VideoFrame object which refers to the given array/tensor.
 
@@ -1292,7 +1305,7 @@ class Muxer:
     """
 
     def __init__(self, dst: str | Path, /, *, format: str | None = None) -> None:
-        self._muxer = _libspdl.muxer(str(dst), format=format)
+        self._muxer: "_libspdl.Muxer" = _libspdl.muxer(str(dst), format=format)
         self._open = False
 
     @overload
@@ -1367,7 +1380,7 @@ def add_remux_stream(self, codec: "AudioCodec | VideoCodec") -> None:
             for packets in demuxer.streaming_demux(num_packets=5):
                 muxer.write(0, packets)
         """
-        self._muxer.add_remux_stream(codec)
+        self._muxer.add_remux_stream(codec)  # pyre-ignore[6]
 
     def open(self, muxer_config: dict[str, str] | None = None) -> "Muxer":
         """Open the muxer (output file) for writing.
@@ -1404,7 +1417,7 @@ def write(self, stream_index: int, packets: "AudioPackets | VideoPackets") -> No
             stream_index: The stream to write to.
             packets: Audio/video data.
         """
-        self._muxer.write(stream_index, packets)
+        self._muxer.write(stream_index, packets)  # pyre-ignore[6]
 
     def flush(self) -> None:
         """Notify the muxer that all the streams are written.