diff --git a/mo/email.py b/mo/email.py
index 0c8ea964a4f0eaf56842a565fd065bba805162ab..174e05420e90d695eee7d93876b54681c3252420 100644
--- a/mo/email.py
+++ b/mo/email.py
@@ -196,7 +196,7 @@ error_tbf = token_bucket.Limiter(rate=1/300, capacity=10, storage=token_bucket.M
 errors_supressed = 0
 
 
-def send_internal_error_email(place: str, attrs: Mapping[str, Optional[str]], exc_info: ExceptionInfo):
+def send_internal_error_email(place: str, attrs: Mapping[str, Optional[str]], exc_info: ExceptionInfo) -> None:
     errors_to = getattr(config, 'MAIL_ERRORS_TO', None)
     if errors_to is None:
         return