diff --git a/src/main/python/ttconv/isd.py b/src/main/python/ttconv/isd.py
index ef7ef74a..77bde2c1 100644
--- a/src/main/python/ttconv/isd.py
+++ b/src/main/python/ttconv/isd.py
@@ -184,7 +184,7 @@ def _make_absolute(
 
     return (begin_time, end_time)
 
-  def _region_always_has_background(region: typing.Type[model.Region]):
+  def _region_always_has_background(region: typing.Type[model.Region]) -> bool:
 
     if region.get_style(styles.StyleProperties.Opacity) == 0:
       return False