diff --git a/mypy.ini b/mypy.ini
index 6d7863b627e..c1d174f4595 100644
--- a/mypy.ini
+++ b/mypy.ini
@@ -4,6 +4,7 @@ files = torchvision
 show_error_codes = True
 pretty = True
 allow_redefinition = True
+no_implicit_optional = True
 warn_redundant_casts = True
 
 [mypy-torchvision.prototype.features.*]
diff --git a/torchvision/datasets/utils.py b/torchvision/datasets/utils.py
index 30506b3fc79..32839163b7b 100644
--- a/torchvision/datasets/utils.py
+++ b/torchvision/datasets/utils.py
@@ -458,7 +458,7 @@ def iterable_to_str(iterable: Iterable) -> str:
 def verify_str_arg(
     value: T,
     arg: Optional[str] = None,
-    valid_values: Iterable[T] = None,
+    valid_values: Optional[Iterable[T]] = None,
     custom_msg: Optional[str] = None,
 ) -> T:
     if not isinstance(value, torch._six.string_classes):