diff --git a/returnn/tf/util/data.py b/returnn/tf/util/data.py
index 70281cb8f1..d0a549461e 100644
--- a/returnn/tf/util/data.py
+++ b/returnn/tf/util/data.py
@@ -3298,6 +3298,18 @@ def get_sequence_mask_broadcast(self, axis=None):
         assert seq_mask.get_shape().ndims == self.batch_ndim
     return seq_mask
 
+  def copy_masked(self, mask_value):
+    """
+    :param float|int mask_value:
+    :rtype: Data
+    """
+    assert self.placeholder is not None
+    from .basic import mask_dyn_seq_len_nd
+    dyn_axes = [axis for axis, dim in enumerate(self.dim_tags) if not dim.is_batch_dim() and dim.dimension is None]
+    res = self.copy()
+    res.placeholder = mask_dyn_seq_len_nd(self, pad_value=mask_value, axes=dyn_axes)
+    return res
+
   def get_batch_dim(self):
     """
     :rtype: tf.Tensor|int