diff --git a/seqio/dataset_providers.py b/seqio/dataset_providers.py
index 66313b67..537f7266 100644
--- a/seqio/dataset_providers.py
+++ b/seqio/dataset_providers.py
@@ -2244,7 +2244,9 @@ class MixtureRegistry(DatasetProviderRegistry):
   def add(
       cls,
       name,
-      tasks,
+      tasks: Union[
+          Sequence[SubtaskOrName], Sequence[Tuple[SubtaskOrName, MixtureRate]]
+      ],
       default_rate=None,
       mixture_cls: Type[Mixture] = Mixture,
       **kwargs,