Iterate las files to Add Surface Information

[SteveHallSES]

I'm trying to run 'Add Surface Information' to populate elevation data to a polygon from 100s of las files inModel Builder.

However it doesn't seem to matter whether I use Files or Datasets as an iterator,  I can't connect the dataset output to the input of the 'Add Surface Info' tool. The only option I get is to use it as a precondition or Environment.

Using 'Exttract LAS' or 'Collect Values' as intermediates doesn't help. The tool works fine on a single LAS file.

What am I doing wrong?