diff --git a/tildes/mypy.ini b/tildes/mypy.ini
index 18aa8b0..4ac6f8b 100644
--- a/tildes/mypy.ini
+++ b/tildes/mypy.ini
@@ -2,3 +2,4 @@
 mypy_path = /opt/tildes/stubs/
 disallow_untyped_defs = true
 ignore_missing_imports = true
+no_implicit_optional = true
diff --git a/tildes/tildes/models/group/group.py b/tildes/tildes/models/group/group.py
index aed3f22..d970545 100644
--- a/tildes/tildes/models/group/group.py
+++ b/tildes/tildes/models/group/group.py
@@ -62,7 +62,7 @@ class Group(DatabaseModel):
         """Order groups by their string representation."""
         return str(self) < str(other)
 
-    def __init__(self, path: str, short_desc: str = None) -> None:
+    def __init__(self, path: str, short_desc: Optional[str] = None) -> None:
         """Create a new group."""
         self.path = Ltree(path)
         self.short_description = short_desc
diff --git a/tildes/tildes/schemas/fields.py b/tildes/tildes/schemas/fields.py
index ef59e54..2f43ebd 100644
--- a/tildes/tildes/schemas/fields.py
+++ b/tildes/tildes/schemas/fields.py
@@ -16,7 +16,9 @@ from tildes.lib.string import simplify_string
 class Enum(Field):
     """Field for a native Python Enum (or subclasses)."""
 
-    def __init__(self, enum_class: Type = None, *args: Any, **kwargs: Any) -> None:
+    def __init__(
+        self, enum_class: Optional[Type] = None, *args: Any, **kwargs: Any
+    ) -> None:
         """Initialize the field with an optional enum class."""
         super().__init__(*args, **kwargs)
         self._enum_class = enum_class
@@ -77,7 +79,7 @@ class Markdown(Field):
 
     DEFAULT_MAX_LENGTH = 50_000
 
-    def __init__(self, max_length: int = None, **kwargs: Any) -> None:
+    def __init__(self, max_length: Optional[int] = None, **kwargs: Any) -> None:
         """Initialize the field with a length validator."""
         if not max_length:
             max_length = self.DEFAULT_MAX_LENGTH
@@ -115,7 +117,7 @@ class SimpleString(Field):
 
     DEFAULT_MAX_LENGTH = 200
 
-    def __init__(self, max_length: int = None, **kwargs: Any) -> None:
+    def __init__(self, max_length: Optional[int] = None, **kwargs: Any) -> None:
         """Initialize the field with a length validator."""
         if not max_length:
             max_length = self.DEFAULT_MAX_LENGTH
diff --git a/tildes/tildes/views/user.py b/tildes/tildes/views/user.py
index e7d297b..542344e 100644
--- a/tildes/tildes/views/user.py
+++ b/tildes/tildes/views/user.py
@@ -1,6 +1,6 @@
 """Views related to a specific user."""
 
-from typing import List, Union
+from typing import List, Optional, Union
 
 from marshmallow.fields import String
 from marshmallow.validate import OneOf
@@ -67,7 +67,11 @@ def _get_user_recent_activity(
     {"post_type": String(load_from="type", validate=OneOf(("topic", "comment")))}
 )
 def get_user(
-    request: Request, after: str, before: str, per_page: int, post_type: str = None
+    request: Request,
+    after: str,
+    before: str,
+    per_page: int,
+    post_type: Optional[str] = None,
 ) -> dict:
     """Generate the main user history page."""
     user = request.context