diff --git a/pyhap/iid_manager.py b/pyhap/iid_manager.py
index 20ce92c6..9bee16f8 100644
--- a/pyhap/iid_manager.py
+++ b/pyhap/iid_manager.py
@@ -9,6 +9,8 @@
     from .characteristic import Characteristic
     from .service import Service
 
+    ServiceOrCharType = Union[Service, Characteristic]
+
 
 class IIDManager:
     """Maintains a mapping between Service/Characteristic objects and IIDs."""
@@ -16,10 +18,10 @@ class IIDManager:
     def __init__(self) -> None:
         """Initialize an empty instance."""
         self.counter = 0
-        self.iids: Dict[Union[Service, Characteristic], int] = {}
-        self.objs: Dict[int, Union[Service, Characteristic]] = {}
+        self.iids: Dict["ServiceOrCharType", int] = {}
+        self.objs: Dict[int, "ServiceOrCharType"] = {}
 
-    def assign(self, obj: Union[Service, Characteristic]) -> None:
+    def assign(self, obj: "ServiceOrCharType") -> None:
         """Assign an IID to given object. Print warning if already assigned.
 
         :param obj: The object that will be assigned an IID.
@@ -38,7 +40,7 @@ def assign(self, obj: Union[Service, Characteristic]) -> None:
         self.iids[obj] = iid
         self.objs[iid] = obj
 
-    def get_iid_for_obj(self, obj: Union[Service, Characteristic]) -> int:
+    def get_iid_for_obj(self, obj: "ServiceOrCharType") -> int:
         """Get the IID for the given object.
 
         Override this method to provide custom IID assignment.
@@ -46,15 +48,15 @@ def get_iid_for_obj(self, obj: Union[Service, Characteristic]) -> int:
         self.counter += 1
         return self.counter
 
-    def get_obj(self, iid: int) -> Union[Service, Characteristic]:
+    def get_obj(self, iid: int) -> "ServiceOrCharType":
         """Get the object that is assigned the given IID."""
         return self.objs.get(iid)
 
-    def get_iid(self, obj: Union[Service, Characteristic]) -> int:
+    def get_iid(self, obj: "ServiceOrCharType") -> int:
         """Get the IID assigned to the given object."""
         return self.iids.get(obj)
 
-    def remove_obj(self, obj: Union[Service, Characteristic]) -> Optional[int]:
+    def remove_obj(self, obj: "ServiceOrCharType") -> Optional[int]:
         """Remove an object from the IID list."""
         iid = self.iids.pop(obj, None)
         if iid is None:
@@ -63,7 +65,7 @@ def remove_obj(self, obj: Union[Service, Characteristic]) -> Optional[int]:
         del self.objs[iid]
         return iid
 
-    def remove_iid(self, iid: int) -> Optional[Union[Service, Characteristic]]:
+    def remove_iid(self, iid: int) -> Optional["ServiceOrCharType"]:
         """Remove an object with an IID from the IID list."""
         obj = self.objs.pop(iid, None)
         if obj is None: