diff --git a/odxtools/isotp_state_machine.py b/odxtools/isotp_state_machine.py
index f6591651..75e4b4df 100644
--- a/odxtools/isotp_state_machine.py
+++ b/odxtools/isotp_state_machine.py
@@ -55,7 +55,7 @@ def decode_rx_frame(self, rx_id: int, data: bytes) -> Iterable[Tuple[int, bytes]
             return  # unknown CAN ID
 
         # decode the isotp segment
-        frame_type, _ = bitstruct.unpack("u4", data)
+        frame_type, _ = bitstruct.unpack("u4u4", data)
         assert isinstance(frame_type, int)
 
         telegram_len = None