diff --git a/backoff/_decorator.py b/backoff/_decorator.py
index 6cf9e17..92dee1b 100644
--- a/backoff/_decorator.py
+++ b/backoff/_decorator.py
@@ -30,9 +30,9 @@ def on_predicate(wait_gen: _WaitGenerator,
                  max_tries: Optional[_MaybeCallable[int]] = None,
                  max_time: Optional[_MaybeCallable[float]] = None,
                  jitter: Union[_Jitterer, None] = full_jitter,
-                 on_success: Union[_Handler, Iterable[_Handler]] = None,
-                 on_backoff: Union[_Handler, Iterable[_Handler]] = None,
-                 on_giveup: Union[_Handler, Iterable[_Handler]] = None,
+                 on_success: Union[_Handler, Iterable[_Handler], None] = None,
+                 on_backoff: Union[_Handler, Iterable[_Handler], None] = None,
+                 on_giveup: Union[_Handler, Iterable[_Handler], None] = None,
                  logger: _MaybeLogger = 'backoff',
                  backoff_log_level: int = logging.INFO,
                  giveup_log_level: int = logging.ERROR,
@@ -127,9 +127,9 @@ def on_exception(wait_gen: _WaitGenerator,
                  max_time: Optional[_MaybeCallable[float]] = None,
                  jitter: Union[_Jitterer, None] = full_jitter,
                  giveup: _Predicate[Exception] = lambda e: False,
-                 on_success: Union[_Handler, Iterable[_Handler]] = None,
-                 on_backoff: Union[_Handler, Iterable[_Handler]] = None,
-                 on_giveup: Union[_Handler, Iterable[_Handler]] = None,
+                 on_success: Union[_Handler, Iterable[_Handler], None] = None,
+                 on_backoff: Union[_Handler, Iterable[_Handler], None] = None,
+                 on_giveup: Union[_Handler, Iterable[_Handler], None] = None,
                  raise_on_giveup: bool = True,
                  logger: _MaybeLogger = 'backoff',
                  backoff_log_level: int = logging.INFO,
diff --git a/backoff/_typing.py b/backoff/_typing.py
index 67a624d..20446d4 100644
--- a/backoff/_typing.py
+++ b/backoff/_typing.py
@@ -1,11 +1,8 @@
 # coding:utf-8
 import logging
 import sys
-from typing import (Any, Callable, Dict, Generator, Sequence, Tuple, Union,
-                    TypeVar)
-
-
-details_kwargs = {"total": False}
+from typing import (Any, Callable, Coroutine, Dict, Generator, Sequence, Tuple,
+                    TypeVar, Union)
 
 if sys.version_info >= (3, 8):  # pragma: no cover
     from typing import TypedDict
@@ -14,8 +11,9 @@
     try:
         from typing_extensions import TypedDict
     except ImportError:
-        TypedDict = Dict[str, Any]
-        del details_kwargs["total"]
+        class TypedDict(dict):
+            def __init_subclass__(cls, **kwargs: Any) -> None:
+                return super().__init_subclass__()
 
 
 class _Details(TypedDict):
@@ -26,7 +24,7 @@ class _Details(TypedDict):
     elapsed: float
 
 
-class Details(_Details, **details_kwargs):
+class Details(_Details, total=False):
     wait: float  # present in the on_backoff handler case for either decorator
     value: Any  # present in the on_predicate decorator case
 
@@ -34,7 +32,10 @@ class Details(_Details, **details_kwargs):
 T = TypeVar("T")
 
 _CallableT = TypeVar('_CallableT', bound=Callable[..., Any])
-_Handler = Callable[[Details], None]
+_Handler = Union[
+    Callable[[Details], None],
+    Callable[[Details], Coroutine[Any, Any, None]],
+]
 _Jitterer = Callable[[float], float]
 _MaybeCallable = Union[T, Callable[[], T]]
 _MaybeLogger = Union[str, logging.Logger, None]