diff --git a/mypy.ini b/mypy.ini
index dca41f8c6b..4c368fcd94 100644
--- a/mypy.ini
+++ b/mypy.ini
@@ -17,4 +17,6 @@
   strict_equality = True
   strict_optional = True
   no_implicit_optional =  True
-  no_implicit_reexport = True
\ No newline at end of file
+  no_implicit_reexport = True
+  # https://mypy.readthedocs.io/en/stable/running_mypy.html#follow-imports
+  follow_imports = silent
diff --git a/opentelemetry-sdk/src/opentelemetry/sdk/util/__init__.pyi b/opentelemetry-sdk/src/opentelemetry/sdk/util/__init__.pyi
index e949ad4ebc..55042fcf0e 100644
--- a/opentelemetry-sdk/src/opentelemetry/sdk/util/__init__.pyi
+++ b/opentelemetry-sdk/src/opentelemetry/sdk/util/__init__.pyi
@@ -49,8 +49,8 @@ class BoundedList(Sequence[_T]):
     @overload
     def __getitem__(self, s: slice) -> Sequence[_T]: ...
     def __len__(self) -> int: ...
-    def append(self, item: _T): ...
-    def extend(self, seq: Sequence[_T]): ...
+    def append(self, item: _T) -> None: ...
+    def extend(self, seq: Sequence[_T]) -> None: ...
     @classmethod
     def from_seq(cls, maxlen: int, seq: Iterable[_T]) -> BoundedList[_T]: ...  # pylint: disable=undefined-variable