diff --git a/crates/nucleus-test-sdk/src/serial.rs b/crates/nucleus-test-sdk/src/serial.rs
index f247a05..81f451d 100644
--- a/crates/nucleus-test-sdk/src/serial.rs
+++ b/crates/nucleus-test-sdk/src/serial.rs
@@ -62,7 +62,12 @@ impl Serial {
                 match p.read(&mut buf) {
                     Ok(0) => Ok(None),
                     Ok(_) => Ok(Some(buf[0])),
-                    Err(e) if e.kind() == std::io::ErrorKind::TimedOut => Ok(None),
+                    Err(e)
+                        if e.kind() == std::io::ErrorKind::WouldBlock
+                            || e.kind() == std::io::ErrorKind::TimedOut =>
+                    {
+                        Ok(None)
+                    }
                     Err(e) => Err(e),
                 }
             }
@@ -105,4 +110,118 @@ mod tests {
         let mut s = Serial::open_tcp(&addr.to_string()).unwrap();
         assert_eq!(s.read_byte(Duration::from_millis(50)).unwrap(), None);
     }
+
+    /// A `serialport::SerialPort` stub whose `read` always returns
+    /// `ErrorKind::WouldBlock` — some serialport backends report a timed-out
+    /// read this way instead of `ErrorKind::TimedOut`.
+    struct WouldBlockPort;
+
+    impl std::io::Read for WouldBlockPort {
+        fn read(&mut self, _buf: &mut [u8]) -> std::io::Result<usize> {
+            Err(std::io::Error::from(std::io::ErrorKind::WouldBlock))
+        }
+    }
+
+    impl std::io::Write for WouldBlockPort {
+        fn write(&mut self, buf: &[u8]) -> std::io::Result<usize> {
+            Ok(buf.len())
+        }
+        fn flush(&mut self) -> std::io::Result<()> {
+            Ok(())
+        }
+    }
+
+    impl serialport::SerialPort for WouldBlockPort {
+        fn name(&self) -> Option<String> {
+            None
+        }
+        fn baud_rate(&self) -> serialport::Result<u32> {
+            Ok(115200)
+        }
+        fn data_bits(&self) -> serialport::Result<serialport::DataBits> {
+            Ok(serialport::DataBits::Eight)
+        }
+        fn flow_control(&self) -> serialport::Result<serialport::FlowControl> {
+            Ok(serialport::FlowControl::None)
+        }
+        fn parity(&self) -> serialport::Result<serialport::Parity> {
+            Ok(serialport::Parity::None)
+        }
+        fn stop_bits(&self) -> serialport::Result<serialport::StopBits> {
+            Ok(serialport::StopBits::One)
+        }
+        fn timeout(&self) -> Duration {
+            Duration::from_millis(50)
+        }
+        fn set_baud_rate(&mut self, _baud_rate: u32) -> serialport::Result<()> {
+            Ok(())
+        }
+        fn set_data_bits(&mut self, _data_bits: serialport::DataBits) -> serialport::Result<()> {
+            Ok(())
+        }
+        fn set_flow_control(
+            &mut self,
+            _flow_control: serialport::FlowControl,
+        ) -> serialport::Result<()> {
+            Ok(())
+        }
+        fn set_parity(&mut self, _parity: serialport::Parity) -> serialport::Result<()> {
+            Ok(())
+        }
+        fn set_stop_bits(&mut self, _stop_bits: serialport::StopBits) -> serialport::Result<()> {
+            Ok(())
+        }
+        fn set_timeout(&mut self, _timeout: Duration) -> serialport::Result<()> {
+            Ok(())
+        }
+        fn write_request_to_send(&mut self, _level: bool) -> serialport::Result<()> {
+            Ok(())
+        }
+        fn write_data_terminal_ready(&mut self, _level: bool) -> serialport::Result<()> {
+            Ok(())
+        }
+        fn read_clear_to_send(&mut self) -> serialport::Result<bool> {
+            Ok(false)
+        }
+        fn read_data_set_ready(&mut self) -> serialport::Result<bool> {
+            Ok(false)
+        }
+        fn read_ring_indicator(&mut self) -> serialport::Result<bool> {
+            Ok(false)
+        }
+        fn read_carrier_detect(&mut self) -> serialport::Result<bool> {
+            Ok(false)
+        }
+        fn bytes_to_read(&self) -> serialport::Result<u32> {
+            Ok(0)
+        }
+        fn bytes_to_write(&self) -> serialport::Result<u32> {
+            Ok(0)
+        }
+        fn clear(&self, _buffer_to_clear: serialport::ClearBuffer) -> serialport::Result<()> {
+            Ok(())
+        }
+        fn try_clone(&self) -> serialport::Result<Box<dyn serialport::SerialPort>> {
+            Err(serialport::Error::new(
+                serialport::ErrorKind::Unknown,
+                "clone not supported in test stub",
+            ))
+        }
+        fn set_break(&self) -> serialport::Result<()> {
+            Ok(())
+        }
+        fn clear_break(&self) -> serialport::Result<()> {
+            Ok(())
+        }
+    }
+
+    #[test]
+    fn port_read_byte_treats_would_block_as_a_timeout_not_an_error() {
+        // Issue #51: the `Port` branch only caught `ErrorKind::TimedOut`,
+        // unlike the `Tcp` branch's `TimedOut | WouldBlock`. Some
+        // serialport backends report a timed-out read as `WouldBlock`,
+        // which used to surface as `Err` here instead of `Ok(None)`.
+        let mut s = Serial::Port(Box::new(WouldBlockPort));
+        assert_eq!(s.read_byte(Duration::from_millis(10)).unwrap(), None);
+    }
 }