diff --git a/nion/utils/Stream.py b/nion/utils/Stream.py
index 6230c8d..920e0a4 100644
--- a/nion/utils/Stream.py
+++ b/nion/utils/Stream.py
@@ -561,3 +561,9 @@ def finalize(task: typing.Optional[asyncio.Task[None]]) -> None:
 
     async def next_value_change(self) -> ValueChange[T]:
         return await self.__event_queue.get()
+
+
+class StaleStreamException(Exception):
+    """Exception class raised when a stream has gone stale (disconnected)."""
+
+    pass
\ No newline at end of file