diff --git a/src/api/categories.py b/src/api/categories.py
index da5c142..549c38d 100644
--- a/src/api/categories.py
+++ b/src/api/categories.py
@@ -10,7 +10,7 @@ from src.api.auth.dependencies import LoggedInDep
 from src.db.models.category import Category
 from src.db.models.event import Event
 from src.db.models.user import User
-from src.utils.db import MissingIdError, expr, update_document
+from src.utils.db import MissingIdError, UnfetchedLinkError, expr, update_document
 from src.utils.logging import get_logger
 
 from .auth import CurrentUserDep
@@ -42,6 +42,7 @@ class _BaseCategoryData(BaseModel):
 class CategoryData(_BaseCategoryData):
     """Data about a category sent to the user."""
 
+    id: PydanticObjectId
     owner_user_id: PydanticObjectId
     created_at: datetime
 
@@ -51,10 +52,16 @@ class CategoryData(_BaseCategoryData):
         if category.id is None:
             raise MissingIdError(category)
 
+        if isinstance(category.user, Link):
+            raise UnfetchedLinkError(category.user)
+        if category.user.id is None:
+            raise MissingIdError(category.user)
+
         return cls(
+            id=category.id,
             name=category.name,
             color=Color(category.color),
-            owner_user_id=category.id,
+            owner_user_id=category.user.id,
             created_at=category.created_at,
         )