diff --git a/proto/marshal/marshal.py b/proto/marshal/marshal.py
index bee1cd6..d278421 100644
--- a/proto/marshal/marshal.py
+++ b/proto/marshal/marshal.py
@@ -191,7 +191,7 @@ def to_python(self, proto_type, value, *, absent: bool = None):
         # See https://github.com/protocolbuffers/protobuf/issues/16596
         # We need to look up the name of the type in compat.map_composite_type_names
         # as class `MessageMapContainer` is no longer exposed
-        # This is done to avoid taking a breaking change in proto-plus
+        # This is done to avoid taking a breaking change in proto-plus.
         if (
             value_type in compat.map_composite_types
             or value_type.__name__ in compat.map_composite_type_names