Filling voids under new subfloor

Hi everyone,
I'm covering 120-year-old wood flooring with a new subfloor and wood flooring. The floor's support is solid but the old wood on top has some deep gouges. The flooring is very thick so there are no holes yet so we're just layering on top.

That will create voids of up to 1/2" deep under the new subfloor. What is the best filler I can use to fill those voids?

Thanks!