diff --git a/tildes/tildes/schemas/fields.py b/tildes/tildes/schemas/fields.py
index d7fa9b7..909b44e 100644
--- a/tildes/tildes/schemas/fields.py
+++ b/tildes/tildes/schemas/fields.py
@@ -8,7 +8,7 @@ from typing import Any, Optional, Type
 
 from marshmallow.exceptions import ValidationError
 from marshmallow.fields import Field, String
-from marshmallow.validate import Length, Regexp
+from marshmallow.validate import Length, OneOf, Regexp
 import sqlalchemy_utils
 
 from tildes.lib.datetime import SimpleHoursPeriod
@@ -148,3 +148,11 @@ class Ltree(Field):
             return sqlalchemy_utils.Ltree(value.lower())
         except (TypeError, ValueError):
             raise ValidationError("Invalid path")
+
+
+class PostType(String):
+    """Field for selecting a type of post (topic or comment)."""
+
+    def __init__(self, **kwargs: Any):
+        """Initialize the field with a "one of" validator."""
+        super().__init__(validate=OneOf(("topic", "comment")), **kwargs)
diff --git a/tildes/tildes/views/bookmarks.py b/tildes/tildes/views/bookmarks.py
index 22eeb0b..3c4f313 100644
--- a/tildes/tildes/views/bookmarks.py
+++ b/tildes/tildes/views/bookmarks.py
@@ -1,9 +1,7 @@
 """Views relating to bookmarks."""
 
-from typing import Optional, Type, Union
+from typing import Type, Union
 
-from marshmallow.fields import String
-from marshmallow.validate import OneOf
 from pyramid.request import Request
 from pyramid.view import view_config
 from sqlalchemy.sql import desc
@@ -11,24 +9,15 @@ from webargs.pyramidparser import use_kwargs
 
 from tildes.models.comment import Comment, CommentBookmark
 from tildes.models.topic import Topic, TopicBookmark
+from tildes.schemas.fields import PostType
 from tildes.schemas.listing import PaginatedListingSchema
 
 
 @view_config(route_name="bookmarks", renderer="bookmarks.jinja2")
 @use_kwargs(PaginatedListingSchema)
-@use_kwargs(
-    {
-        "post_type": String(
-            load_from="type", validate=OneOf(("topic", "comment")), missing="topic"
-        )
-    }
-)
+@use_kwargs({"post_type": PostType(load_from="type", missing="topic")})
 def get_bookmarks(
-    request: Request,
-    after: str,
-    before: str,
-    per_page: int,
-    post_type: Optional[str] = None,
+    request: Request, after: str, before: str, per_page: int, post_type: str
 ) -> dict:
     """Generate the bookmarks page."""
     # pylint: disable=unused-argument
diff --git a/tildes/tildes/views/user.py b/tildes/tildes/views/user.py
index 730e246..b4bd7ab 100644
--- a/tildes/tildes/views/user.py
+++ b/tildes/tildes/views/user.py
@@ -5,8 +5,6 @@
 
 from typing import List, Optional, Union
 
-from marshmallow.fields import String
-from marshmallow.validate import OneOf
 from pyramid.request import Request
 from pyramid.view import view_config
 from sqlalchemy.sql.expression import desc
@@ -16,6 +14,7 @@ from tildes.enums import CommentLabelOption
 from tildes.models.comment import Comment
 from tildes.models.topic import Topic
 from tildes.models.user import User, UserInviteCode
+from tildes.schemas.fields import PostType
 from tildes.schemas.listing import PaginatedListingSchema
 
 
@@ -67,9 +66,7 @@ def _get_user_recent_activity(
 
 @view_config(route_name="user", renderer="user.jinja2")
 @use_kwargs(PaginatedListingSchema())
-@use_kwargs(
-    {"post_type": String(load_from="type", validate=OneOf(("topic", "comment")))}
-)
+@use_kwargs({"post_type": PostType(load_from="type")})
 def get_user(
     request: Request,
     after: str,