diff --git a/betterproto/__init__.py b/betterproto/__init__.py
index ee36708..a9bc4b6 100644
--- a/betterproto/__init__.py
+++ b/betterproto/__init__.py
@@ -420,11 +420,14 @@ class Message(ABC):
     register the message fields which get used by the serializers and parsers
     to go between Python, binary and JSON protobuf message representations.
     """
+    _serialized_on_wire: bool
+    _unknown_fields: bytes
+    _group_map: Dict[str, dict]
 
     def __post_init__(self) -> None:
         # Set a default value for each field in the class after `__init__` has
         # already been run.
-        group_map = {"fields": {}, "groups": {}}
+        group_map: Dict[str, dict] = {"fields": {}, "groups": {}}
         for field in dataclasses.fields(self):
             meta = FieldMetadata.get(field)
 
@@ -518,7 +521,7 @@ class Message(ABC):
                 else:
                     for item in value:
                         output += _serialize_single(
-                            meta.number, meta.proto_type, item, wraps=meta.wraps
+                            meta.number, meta.proto_type, item, wraps=meta.wraps or ""
                         )
             elif isinstance(value, dict):
                 for k, v in value.items():
@@ -532,7 +535,7 @@ class Message(ABC):
                     meta.proto_type,
                     value,
                     serialize_empty=serialize_empty,
-                    wraps=meta.wraps,
+                    wraps=meta.wraps or "",
                 )
 
         return output + self._unknown_fields
@@ -702,14 +705,14 @@ class Message(ABC):
         for field in dataclasses.fields(self):
             meta = FieldMetadata.get(field)
             v = getattr(self, field.name)
-            cased_name = casing(field.name).rstrip("_")
+            cased_name = casing(field.name).rstrip("_") # type: ignore
             if meta.proto_type == "message":
                 if isinstance(v, datetime):
                     if v != DATETIME_ZERO:
-                        output[cased_name] = _Timestamp.to_json(v)
+                        output[cased_name] = _Timestamp.timestamp_to_json(v)
                 elif isinstance(v, timedelta):
                     if v != timedelta(0):
-                        output[cased_name] = _Duration.to_json(v)
+                        output[cased_name] = _Duration.delta_to_json(v)
                 elif meta.wraps:
                     if v is not None:
                         output[cased_name] = v
@@ -738,7 +741,7 @@ class Message(ABC):
                     else:
                         output[cased_name] = b64encode(v).decode("utf8")
                 elif meta.proto_type == TYPE_ENUM:
-                    enum_values = list(self._cls_for(field))
+                    enum_values = list(self._cls_for(field)) # type: ignore
                     if isinstance(v, list):
                         output[cased_name] = [enum_values[e].name for e in v]
                     else:
@@ -853,7 +856,7 @@ class _Duration(Message):
         return timedelta(seconds=self.seconds, microseconds=self.nanos / 1e3)
 
     @staticmethod
-    def to_json(delta: timedelta) -> str:
+    def delta_to_json(delta: timedelta) -> str:
         parts = str(delta.total_seconds()).split(".")
         if len(parts) > 1:
             while len(parts[1]) not in [3, 6, 9]:
@@ -876,7 +879,7 @@ class _Timestamp(Message):
         return datetime.fromtimestamp(ts, tz=timezone.utc)
 
     @staticmethod
-    def to_json(dt: datetime) -> str:
+    def timestamp_to_json(dt: datetime) -> str:
         nanos = dt.microsecond * 1e3
         copy = dt.replace(microsecond=0, tzinfo=None)
         result = copy.isoformat()
@@ -899,12 +902,15 @@ class _WrappedMessage(Message):
     Google protobuf wrapper types base class. JSON representation is just the
     value itself.
     """
-    def to_dict(self) -> Any:
+    value: Any
+
+    def to_dict(self, casing: Casing = Casing.CAMEL) -> Any:
         return self.value
 
-    def from_dict(self, value: Any) -> None:
+    def from_dict(self: T, value: Any) -> T:
         if value is not None:
             self.value = value
+        return self
 
 
 @dataclasses.dataclass
@@ -952,7 +958,7 @@ class _BytesValue(_WrappedMessage):
     value: bytes = bytes_field(1)
 
 
-def _get_wrapper(proto_type: str) -> _WrappedMessage:
+def _get_wrapper(proto_type: str) -> Type:
     """Get the wrapper message class for a wrapped type."""
     return {
         TYPE_BOOL: _BoolValue,