diff --git a/python/uds.py b/python/uds.py
index 9b25dffe6..f60720941 100644
--- a/python/uds.py
+++ b/python/uds.py
@@ -473,6 +473,7 @@ class IsoTpMessage():
     # assert len(rx_data) == self.max_len, f"isotp - rx: invalid CAN frame length: {len(rx_data)}"
 
     if rx_data[0] >> 4 == ISOTP_FRAME_TYPE.SINGLE:
+      assert not self.rx_done, "isotp - rx: unexpected single frame"
       self.rx_len = rx_data[0] & 0x0F
       assert self.rx_len < self.max_len, f"isotp - rx: invalid single frame length: {self.rx_len}"
       self.rx_dat = rx_data[1:1 + self.rx_len]
@@ -483,6 +484,7 @@ class IsoTpMessage():
       return ISOTP_FRAME_TYPE.SINGLE
 
     elif rx_data[0] >> 4 == ISOTP_FRAME_TYPE.FIRST:
+      assert self.rx_dat == b"", "isotp - rx: first frame with data already received"
       self.rx_len = ((rx_data[0] & 0x0F) << 8) + rx_data[1]
       assert self.max_len <= self.rx_len, f"isotp - rx: invalid first frame length: {self.rx_len}"
       self.rx_dat = rx_data[2:]