diff --git a/matemat/db/facade.py b/matemat/db/facade.py
index 0f5d984..a94e420 100644
--- a/matemat/db/facade.py
+++ b/matemat/db/facade.py
@@ -222,7 +222,7 @@ class MatematDatabase(object):
             if verify_password and not compare_digest(crypt.crypt(password, row[0]), row[0]):
                 raise AuthenticationError('Password does not match.')
             # Hash the new touchkey and write it to the database.
-            tkhash: str = crypt.crypt(touchkey, crypt.mksalt(_CRYPT_METHOD)) if touchkey is not None else None
+            tkhash: Optional[str] = crypt.crypt(touchkey, crypt.mksalt(_CRYPT_METHOD)) if touchkey is not None else None
             c.execute('''
             UPDATE users SET touchkey = :tkhash, lastchange = STRFTIME('%s', 'now') WHERE user_id = :user_id
             ''', {
diff --git a/matemat/db/wrapper.py b/matemat/db/wrapper.py
index e930d17..34af430 100644
--- a/matemat/db/wrapper.py
+++ b/matemat/db/wrapper.py
@@ -1,5 +1,5 @@
 
-from typing import Any
+from typing import Any, Optional
 
 import sqlite3
 
@@ -10,7 +10,7 @@ class Transaction(object):
 
     def __init__(self, db: sqlite3.Connection, exclusive: bool = True) -> None:
         self._db: sqlite3.Connection = db
-        self._cursor = None
+        self._cursor: Optional[sqlite3.Cursor] = None
         self._excl = exclusive
         self._is_dummy: bool = False
 
@@ -74,7 +74,7 @@ class DatabaseWrapper(object):
 
     def __init__(self, filename: str) -> None:
         self._filename: str = filename
-        self._sqlite_db: sqlite3.Connection = None
+        self._sqlite_db: Optional[sqlite3.Connection] = None
 
     def __enter__(self) -> 'DatabaseWrapper':
         self.connect()
@@ -84,6 +84,8 @@ class DatabaseWrapper(object):
         self.close()
 
     def transaction(self, exclusive: bool = True) -> Transaction:
+        if self._sqlite_db is None:
+            raise RuntimeError(f'Database connection to {self._filename} is not established.')
         return Transaction(self._sqlite_db, exclusive)
 
     def _setup(self) -> None:
@@ -113,13 +115,15 @@ class DatabaseWrapper(object):
         self._sqlite_db = None
 
     def in_transaction(self) -> bool:
-        return self._sqlite_db.in_transaction
+        return self._sqlite_db is not None and self._sqlite_db.in_transaction
 
     def is_connected(self) -> bool:
         return self._sqlite_db is not None
 
     @property
     def _user_version(self) -> int:
+        if self._sqlite_db is None:
+            raise RuntimeError(f'Database connection to {self._filename} is not established.')
         cursor = self._sqlite_db.cursor()
         cursor.execute('PRAGMA user_version')
         version = int(cursor.fetchone()[0])
@@ -127,5 +131,7 @@ class DatabaseWrapper(object):
 
     @_user_version.setter
     def _user_version(self, version: int) -> None:
+        if self._sqlite_db is None:
+            raise RuntimeError(f'Database connection to {self._filename} is not established.')
         cursor = self._sqlite_db.cursor()
         cursor.execute(f'PRAGMA user_version = {version}')
diff --git a/matemat/exceptions/HttpException.py b/matemat/exceptions/HttpException.py
index 4f792ce..35fefd3 100644
--- a/matemat/exceptions/HttpException.py
+++ b/matemat/exceptions/HttpException.py
@@ -1,11 +1,14 @@
 
+from typing import Optional
+
+
 class HttpException(Exception):
 
     def __init__(self, status: int = 500, title: str = 'An error occurred', message: str = None) -> None:
         super().__init__()
         self.__status: int = status
         self.__title: str = title
-        self.__message: str = message
+        self.__message: Optional[str] = message
 
     @property
     def status(self) -> int:
@@ -16,5 +19,5 @@ class HttpException(Exception):
         return self.__title
 
     @property
-    def message(self) -> str:
+    def message(self) -> Optional[str]:
         return self.__message
diff --git a/matemat/webserver/responses.py b/matemat/webserver/responses.py
index 6f0308f..cf9dbf8 100644
--- a/matemat/webserver/responses.py
+++ b/matemat/webserver/responses.py
@@ -11,7 +11,7 @@ class PageletResponse:
     An instance of this base class will result in an empty 200 OK response.
     """
 
-    def __init__(self, status: int = 200):
+    def __init__(self, status: int = 200) -> None:
         """
         Create an empty response.
 
@@ -26,7 +26,7 @@ class RedirectResponse(PageletResponse):
     response status, and a Location header.
     """
 
-    def __init__(self, location: str):
+    def __init__(self, location: str) -> None:
         """
         Create a redirection response with the given redirection location.
 
@@ -42,7 +42,7 @@ class TemplateResponse(PageletResponse):
     sending the result as response body, with a 200 OK response status.
     """
 
-    def __init__(self, name: str, **kwargs):
+    def __init__(self, name: str, **kwargs) -> None:
         """
         Create a template response with the given template name and arguments.