diff --git a/mypy.ini b/mypy.ini
index eb93645605..b6ebfb5cd6 100644
--- a/mypy.ini
+++ b/mypy.ini
@@ -12,6 +12,7 @@ check_untyped_defs = True
 disallow_untyped_defs = True
 disallow_any_generics = True
 warn_no_return = True
+strict_optional = True
 no_implicit_optional = True
 warn_redundant_casts = True
 warn_unused_ignores = True
@@ -29,16 +30,3 @@ ignore_missing_imports = True
 
 # Warn of unreachable or redundant code.
 warn_unreachable = True
-
-#
-#
-# STRICT OPTIONAL
-#
-#
-
-strict_optional = True
-
-# General exclusions to work on
-
-[mypy-zthumbor.loaders.helpers]
-strict_optional = False
diff --git a/zthumbor/loaders/helpers.py b/zthumbor/loaders/helpers.py
index b595256dec..d85bb802e5 100644
--- a/zthumbor/loaders/helpers.py
+++ b/zthumbor/loaders/helpers.py
@@ -44,4 +44,5 @@ def separate_url_and_source_type(url):
     # type: (Text) -> Tuple[Text, Text]
     THUMBNAIL_URL_PATT = re.compile('^(?P<actual_url>.+)/source_type/(?P<source_type>.+)')
     matches = THUMBNAIL_URL_PATT.match(url)
+    assert matches is not None
     return (matches.group('source_type'), matches.group('actual_url'))