diff --git a/design/mvp/canonical-abi/definitions.py b/design/mvp/canonical-abi/definitions.py
index 2dd0ea0b..2ff543f8 100644
--- a/design/mvp/canonical-abi/definitions.py
+++ b/design/mvp/canonical-abi/definitions.py
@@ -315,20 +315,22 @@ async def default_on_block(f):
   await current_task.acquire()
   return v
 
-async def call_and_handle_blocking(callee):
+class Blocked: pass
+
+async def call_and_handle_blocking(callee, *args) -> Blocked|any:
   blocked = asyncio.Future()
   async def on_block(f):
     if not blocked.done():
-      blocked.set_result(True)
+      blocked.set_result(Blocked())
     else:
       current_task.release()
     v = await f
     await current_task.acquire()
     return v
   async def do_call():
-    await callee(on_block)
+    ret = await callee(*args, on_block)
     if not blocked.done():
-      blocked.set_result(False)
+      blocked.set_result(ret)
     else:
       current_task.release()
   asyncio.create_task(do_call())
@@ -1454,13 +1456,14 @@ async def canon_lower(opts, ft, callee, task, flat_args):
     async def do_call(on_block):
       await callee(task, subtask.on_start, subtask.on_return, on_block)
       [] = subtask.finish()
-    if await call_and_handle_blocking(do_call):
-      subtask.notify_supertask = True
-      task.need_to_drop += 1
-      i = task.inst.async_subtasks.add(subtask)
-      flat_results = [pack_async_result(i, subtask.state)]
-    else:
-      flat_results = [0]
+    match await call_and_handle_blocking(do_call):
+      case Blocked():
+        subtask.notify_supertask = True
+        task.need_to_drop += 1
+        i = task.inst.async_subtasks.add(subtask)
+        flat_results = [pack_async_result(i, subtask.state)]
+      case _:
+        flat_results = [0]
   return flat_results
 
 def pack_async_result(i, state):