diff --git a/src/asyncio_taskpool/pool.py b/src/asyncio_taskpool/pool.py
index c7f4993..232d058 100644
--- a/src/asyncio_taskpool/pool.py
+++ b/src/asyncio_taskpool/pool.py
@@ -150,22 +150,26 @@ class BaseTaskPool:
             raise e
         return task_id
 
-    def _cancel_one(self, task_id: int, msg: str = None) -> None:
+    def _get_running_task(self, task_id: int) -> Task:
         try:
-            task = self._running[task_id]
+            return self._running[task_id]
         except KeyError:
             if self._cancelled.get(task_id):
                 raise exceptions.AlreadyCancelled(f"{self._task_name(task_id)} has already been cancelled")
             if self._ended.get(task_id):
                 raise exceptions.AlreadyFinished(f"{self._task_name(task_id)} has finished running")
             raise exceptions.InvalidTaskID(f"No task with ID {task_id} found in {self}")
-        task.cancel(msg=msg)
+
+    def _cancel_task(self, task_id: int, msg: str = None) -> None:
+        self._get_running_task(task_id).cancel(msg=msg)
 
     def cancel(self, *task_ids: int, msg: str = None) -> None:
-        for task_id in task_ids:
-            self._cancel_one(task_id, msg=msg)
+        tasks = [self._get_running_task(task_id) for task_id in task_ids]
+        for task in tasks:
+            task.cancel(msg=msg)
 
-    def cancel_all(self, msg: str = None) -> None:
+    async def cancel_all(self, msg: str = None) -> None:
+        await self._all_tasks_known_flag.wait()
         for task in self._running.values():
             task.cancel(msg=msg)