diff --git a/src/ibex_bluesky_core/plan_stubs/__init__.py b/src/ibex_bluesky_core/plan_stubs/__init__.py
index 776326f..9d34425 100644
--- a/src/ibex_bluesky_core/plan_stubs/__init__.py
+++ b/src/ibex_bluesky_core/plan_stubs/__init__.py
@@ -1,6 +1,6 @@
 """Core plan stubs."""
 
-from typing import Callable, Generator, ParamSpec, TypeVar
+from typing import Callable, Generator, ParamSpec, TypeVar, cast
 
 import bluesky.plan_stubs as bps
 from bluesky.utils import Msg
@@ -12,9 +12,7 @@
 CALL_SYNC_MSG_KEY = "ibex_bluesky_core_call_sync"
 
 
-def call_sync(
-    func: Callable[P, T], *args: P.args, **kwargs: P.kwargs
-) -> Generator[Msg, None, None]:
+def call_sync(func: Callable[P, T], *args: P.args, **kwargs: P.kwargs) -> Generator[Msg, None, T]:
     """Call a synchronous user function in a plan, and returns the result of that call.
 
     Attempts to guard against the most common pitfalls of naive implementations, for example:
@@ -41,4 +39,4 @@ def call_sync(
 
     """
     yield from bps.clear_checkpoint()
-    return (yield Msg(CALL_SYNC_MSG_KEY, func, *args, **kwargs))
+    return cast(T, (yield Msg(CALL_SYNC_MSG_KEY, func, *args, **kwargs)))
diff --git a/src/ibex_bluesky_core/run_engine/_msg_handlers.py b/src/ibex_bluesky_core/run_engine/_msg_handlers.py
index 02c1a1d..d16bcbb 100644
--- a/src/ibex_bluesky_core/run_engine/_msg_handlers.py
+++ b/src/ibex_bluesky_core/run_engine/_msg_handlers.py
@@ -24,15 +24,13 @@ async def call_sync_handler(msg: Msg) -> Any:  # noqa: ANN401
     loop = get_running_loop()
 
     def _wrapper() -> Any:  # noqa: ANN401
+        nonlocal ret, exc
         try:
-            nonlocal ret
             ret = func(*msg.args, **msg.kwargs)
         except _ExternalFunctionInterrupted:
             pass  # Suppress stack traces from our special interruption exception.
         except BaseException as e:
-            nonlocal exc
             exc = e
-            raise
         finally:
             loop.call_soon_threadsafe(done_event.set)
 
@@ -51,7 +49,7 @@ def _wrapper() -> Any:  # noqa: ANN401
         #   be a rather long-running external function, so would prevent the interrupt from working)
         # - Interrupt but don't actually kill the thread, this leads to a misleading result where
         #   a user gets the shell back but the task is still running in the background
-        # - Hack around with ctypes to inject an injection into the thread running the external
+        # - Hack around with ctypes to inject an exception into the thread running the external
         #   function. This is generally frowned upon as a bad idea, but may be the "least bad"
         #   solution to running potentially-blocking user code within a plan.
         # - Force users to pass in coroutines (which await regularly) or functions which check some