diff --git a/returnn/frontend/types.py b/returnn/frontend/types.py
index b0b2ba8e3d..fae0893ffb 100644
--- a/returnn/frontend/types.py
+++ b/returnn/frontend/types.py
@@ -22,7 +22,7 @@
 
 
 RawTensorTypes = Union[int, float, complex, numpy.number, numpy.ndarray, bool, str]
-ItemKeyType = Union[RawTensorTypes, Tensor, slice, Sequence[RawTensorTypes, Tensor, slice]]
+ItemKeyType = Union[RawTensorTypes, Tensor, slice, Sequence[Union[RawTensorTypes, Tensor, slice]]]
 
 
 class GetModelFunc(Protocol):