diff --git a/src/openklant/components/klantinteracties/api/serializers/klantcontacten.py b/src/openklant/components/klantinteracties/api/serializers/klantcontacten.py
index 1ab1229..7f49422 100644
--- a/src/openklant/components/klantinteracties/api/serializers/klantcontacten.py
+++ b/src/openklant/components/klantinteracties/api/serializers/klantcontacten.py
@@ -209,7 +209,7 @@ class Meta:
             },
         }
 
-    def get_volledige_naam(self, obj):
+    def get_volledige_naam(self, obj) -> str:
         return obj.get_full_name()
 
     @transaction.atomic
diff --git a/src/openklant/components/klantinteracties/api/serializers/partijen.py b/src/openklant/components/klantinteracties/api/serializers/partijen.py
index 4b24670..66f9b24 100644
--- a/src/openklant/components/klantinteracties/api/serializers/partijen.py
+++ b/src/openklant/components/klantinteracties/api/serializers/partijen.py
@@ -115,7 +115,7 @@ class Meta:
             },
         }
 
-    def get_categorie_naam(self, obj):
+    def get_categorie_naam(self, obj) -> str | None:
         if obj.categorie:
             return obj.categorie.naam
 
@@ -301,7 +301,7 @@ class Meta:
             "volledige_naam",
         )
 
-    def get_volledige_naam(self, obj):
+    def get_volledige_naam(self, obj) -> str:
         return obj.get_full_name()
 
 
@@ -333,7 +333,7 @@ class Meta:
             "volledige_naam",
         )
 
-    def get_volledige_naam(self, obj):
+    def get_volledige_naam(self, obj) -> str:
         return obj.get_full_name()
 
     @transaction.atomic