diff --git a/src/phi/async_ldap/mixins.py b/src/phi/async_ldap/mixins.py
index 0ae52aa..1aff860 100644
--- a/src/phi/async_ldap/mixins.py
+++ b/src/phi/async_ldap/mixins.py
@@ -4,10 +4,8 @@ from bonsai.ldapvaluelist import LDAPValueList
 import bonsai.errors
 
 from phi.exceptions import (
-    PhiAttributeMissing,
     PhiEntryDoesNotExist,
     PhiEntryExists,
-    PhiUnauthorized,
     PhiUnexpectedRuntimeValue,
     PhiCannotExecute,
 )
@@ -48,6 +46,8 @@ class Singleton(object):
             name = f"{cls.__name__}-{args[0]}-{id(client)}"
         else:
             name = f"{cls.__name__}-{id(client)}"
+        if not hasattr(cls, "_instances"):
+            cls._instances = dict()
         if name not in cls._instances:
             cls._instances[name] = object.__new__(cls)
         return cls._instances[name]
diff --git a/src/phi/async_ldap/model.py b/src/phi/async_ldap/model.py
index 5e08e68..dec8656 100644
--- a/src/phi/async_ldap/model.py
+++ b/src/phi/async_ldap/model.py
@@ -4,6 +4,9 @@ from bonsai import LDAPEntry
 from multidict import MultiDict
 
 from phi.async_ldap import mixins
+from phi.exceptions import (
+    PhiUnexpectedRuntimeValue,
+)
 
 
 def parse_dn(dn):
@@ -60,7 +63,7 @@ class Service(mixins.Member, mixins.Entry, mixins.Singleton):
     ldap_attributes = ["uid", "ou", "userPassword"]
 
     async def iter_groups(self):  # To be monkeypatched later
-        pass  # pragma: no cover
+        raise NotImplemented
 
     async def groups(self):
         return [g async for g in self.iter_groups()]