diff --git a/betterproto/__init__.py b/betterproto/__init__.py
index 6a53d652f..c1e60ea2b 100644
--- a/betterproto/__init__.py
+++ b/betterproto/__init__.py
@@ -440,7 +440,7 @@ class ProtoClassMetadata:
 
     def __init__(self, cls: Type["Message"]):
         by_field = {}
-        by_group = {}
+        by_group: Dict[str, Set] = {}
         by_field_name = {}
         by_field_number = {}
 
@@ -780,7 +780,7 @@ def FromString(cls: Type[T], data: bytes) -> T:
 
     def to_dict(
         self, casing: Casing = Casing.CAMEL, include_default_values: bool = False
-    ) -> dict:
+    ) -> Dict[str, Any]:
         """
         Returns a dict representation of this message instance which can be
         used to serialize to e.g. JSON. Defaults to camel casing for
diff --git a/betterproto/_types.py b/betterproto/_types.py
index 0ff23e45e..d03432cd0 100644
--- a/betterproto/_types.py
+++ b/betterproto/_types.py
@@ -1,4 +1,8 @@
-from typing import TypeVar
+from typing import TYPE_CHECKING, TypeVar
+
+if TYPE_CHECKING:
+    from . import Message
+    from grpclib._protocols import IProtoMessage
 
 # Bound type variable to allow methods to return `self` of subclasses
 T = TypeVar("T", bound="Message")
diff --git a/betterproto/grpc/grpclib_client.py b/betterproto/grpc/grpclib_client.py
index 7218574b8..7f48fb995 100644
--- a/betterproto/grpc/grpclib_client.py
+++ b/betterproto/grpc/grpclib_client.py
@@ -3,9 +3,10 @@
 import grpclib.const
 from typing import (
     Any,
+    AsyncIterable,
     AsyncIterator,
     Collection,
-    Iterator,
+    Iterable,
     Mapping,
     Optional,
     Tuple,
@@ -23,7 +24,7 @@
 
 _Value = Union[str, bytes]
 _MetadataLike = Union[Mapping[str, _Value], Collection[Tuple[str, _Value]]]
-_MessageSource = Union[Iterator["IProtoMessage"], AsyncIterator["IProtoMessage"]]
+_MessageSource = Union[Iterable["IProtoMessage"], AsyncIterable["IProtoMessage"]]
 
 
 class ServiceStub(ABC):
@@ -160,7 +161,7 @@ async def _stream_stream(
 
     @staticmethod
     async def _send_messages(stream, messages: _MessageSource):
-        if hasattr(messages, "__aiter__"):
+        if isinstance(messages, AsyncIterable):
             async for message in messages:
                 await stream.send_message(message)
         else:
diff --git a/betterproto/plugin.py b/betterproto/plugin.py
index 85fd9057c..ed14e000d 100755
--- a/betterproto/plugin.py
+++ b/betterproto/plugin.py
@@ -6,10 +6,10 @@
 import stringcase
 import sys
 import textwrap
-from typing import List
+from typing import List, Union
+import betterproto
 from betterproto.casing import safe_snake_case
 from betterproto.compile.importing import get_ref_type
-import betterproto
 
 try:
     # betterproto[compiler] specific dependencies
@@ -58,8 +58,8 @@ def py_type(
         raise NotImplementedError(f"Unknown type {descriptor.type}")
 
 
-def get_py_zero(type_num: int) -> str:
-    zero = 0
+def get_py_zero(type_num: int) -> Union[str, float]:
+    zero: Union[str, float] = 0
     if type_num in []:
         zero = 0.0
     elif type_num == 8: