@@ -5,6 +5,7 @@ import asyncio

 import concurrent.futures as concurrent

 import inspect

 import os

+import pickle

 import sys

 import time

 import traceback

@@ -494,6 +495,18 @@ class AsyncRunner(BaseRunner):

             def goal(_):

                 return False

+        if executor is None and not inspect.iscoroutinefunction(learner.function):

+            try:

+                pickle.dumps(learner.function)

+            except pickle.PicklingError:

+                raise ValueError(

+                    "`learner.function` cannot be pickled (is it a lamdba function?)"

+                    " and therefore does not work with the default executor."

+                    " Either make sure the function is pickleble or use an executor"

+                    " that might work with 'hard to pickle'-functions"

+                    " , e.g. `ipyparallel` with `dill`."

+                )

+

         super().__init__(

             learner,

             goal,