diff --git a/ensure/main.py b/ensure/main.py
index 92e28ae..9da788e 100644
--- a/ensure/main.py
+++ b/ensure/main.py
@@ -878,6 +878,12 @@ def f(x: int, y: float) -> float:
 
         >>> ensure.EnsureError: Argument y of type <class 'int'> to
         <function f at 0x109b7c710> does not match annotation type <class 'float'>
+
+    .. warning::
+
+       The current implementation of the decorator might trigger
+       a ``_pickle.PicklingError`` when a decorated function is run through
+       ``multiprocessing``.
     """
 
     if f.__defaults__: