diff --git a/databases/core.py b/databases/core.py
index efa59471..8415b836 100644
--- a/databases/core.py
+++ b/databases/core.py
@@ -129,20 +129,24 @@ async def __aenter__(self) -> "Database":
 
     async def __aexit__(
         self,
-        exc_type: typing.Type[BaseException] = None,
-        exc_value: BaseException = None,
-        traceback: TracebackType = None,
+        exc_type: typing.Optional[typing.Type[BaseException]] = None,
+        exc_value: typing.Optional[BaseException] = None,
+        traceback: typing.Optional[TracebackType] = None,
     ) -> None:
         await self.disconnect()
 
     async def fetch_all(
-        self, query: typing.Union[ClauseElement, str], values: dict = None
+        self,
+        query: typing.Union[ClauseElement, str],
+        values: typing.Optional[dict] = None,
     ) -> typing.List[Record]:
         async with self.connection() as connection:
             return await connection.fetch_all(query, values)
 
     async def fetch_one(
-        self, query: typing.Union[ClauseElement, str], values: dict = None
+        self,
+        query: typing.Union[ClauseElement, str],
+        values: typing.Optional[dict] = None,
     ) -> typing.Optional[Record]:
         async with self.connection() as connection:
             return await connection.fetch_one(query, values)
@@ -150,14 +154,16 @@ async def fetch_one(
     async def fetch_val(
         self,
         query: typing.Union[ClauseElement, str],
-        values: dict = None,
+        values: typing.Optional[dict] = None,
         column: typing.Any = 0,
     ) -> typing.Any:
         async with self.connection() as connection:
             return await connection.fetch_val(query, values, column=column)
 
     async def execute(
-        self, query: typing.Union[ClauseElement, str], values: dict = None
+        self,
+        query: typing.Union[ClauseElement, str],
+        values: typing.Optional[dict] = None,
     ) -> typing.Any:
         async with self.connection() as connection:
             return await connection.execute(query, values)
@@ -169,7 +175,9 @@ async def execute_many(
             return await connection.execute_many(query, values)
 
     async def iterate(
-        self, query: typing.Union[ClauseElement, str], values: dict = None
+        self,
+        query: typing.Union[ClauseElement, str],
+        values: typing.Optional[dict] = None,
     ) -> typing.AsyncGenerator[typing.Mapping, None]:
         async with self.connection() as connection:
             async for record in connection.iterate(query, values):
@@ -232,9 +240,9 @@ async def __aenter__(self) -> "Connection":
 
     async def __aexit__(
         self,
-        exc_type: typing.Type[BaseException] = None,
-        exc_value: BaseException = None,
-        traceback: TracebackType = None,
+        exc_type: typing.Optional[typing.Type[BaseException]] = None,
+        exc_value: typing.Optional[BaseException] = None,
+        traceback: typing.Optional[TracebackType] = None,
     ) -> None:
         async with self._connection_lock:
             assert self._connection is not None
@@ -243,14 +251,18 @@ async def __aexit__(
                 await self._connection.release()
 
     async def fetch_all(
-        self, query: typing.Union[ClauseElement, str], values: dict = None
+        self,
+        query: typing.Union[ClauseElement, str],
+        values: typing.Optional[dict] = None,
     ) -> typing.List[Record]:
         built_query = self._build_query(query, values)
         async with self._query_lock:
             return await self._connection.fetch_all(built_query)
 
     async def fetch_one(
-        self, query: typing.Union[ClauseElement, str], values: dict = None
+        self,
+        query: typing.Union[ClauseElement, str],
+        values: typing.Optional[dict] = None,
     ) -> typing.Optional[Record]:
         built_query = self._build_query(query, values)
         async with self._query_lock:
@@ -259,7 +271,7 @@ async def fetch_one(
     async def fetch_val(
         self,
         query: typing.Union[ClauseElement, str],
-        values: dict = None,
+        values: typing.Optional[dict] = None,
         column: typing.Any = 0,
     ) -> typing.Any:
         built_query = self._build_query(query, values)
@@ -267,7 +279,9 @@ async def fetch_val(
             return await self._connection.fetch_val(built_query, column)
 
     async def execute(
-        self, query: typing.Union[ClauseElement, str], values: dict = None
+        self,
+        query: typing.Union[ClauseElement, str],
+        values: typing.Optional[dict] = None,
     ) -> typing.Any:
         built_query = self._build_query(query, values)
         async with self._query_lock:
@@ -281,7 +295,9 @@ async def execute_many(
             await self._connection.execute_many(queries)
 
     async def iterate(
-        self, query: typing.Union[ClauseElement, str], values: dict = None
+        self,
+        query: typing.Union[ClauseElement, str],
+        values: typing.Optional[dict] = None,
     ) -> typing.AsyncGenerator[typing.Any, None]:
         built_query = self._build_query(query, values)
         async with self.transaction():
@@ -303,7 +319,7 @@ def raw_connection(self) -> typing.Any:
 
     @staticmethod
     def _build_query(
-        query: typing.Union[ClauseElement, str], values: dict = None
+        query: typing.Union[ClauseElement, str], values: typing.Optional[dict] = None
     ) -> ClauseElement:
         if isinstance(query, str):
             query = text(query)
@@ -338,9 +354,9 @@ async def __aenter__(self) -> "Transaction":
 
     async def __aexit__(
         self,
-        exc_type: typing.Type[BaseException] = None,
-        exc_value: BaseException = None,
-        traceback: TracebackType = None,
+        exc_type: typing.Optional[typing.Type[BaseException]] = None,
+        exc_value: typing.Optional[BaseException] = None,
+        traceback: typing.Optional[TracebackType] = None,
     ) -> None:
         """
         Called when exiting `async with database.transaction()`
diff --git a/setup.cfg b/setup.cfg
index 77c8c58d..da1831fd 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -1,6 +1,7 @@
 [mypy]
 disallow_untyped_defs = True
 ignore_missing_imports = True
+no_implicit_optional = True
 
 [tool:isort]
 profile = black