How to call existing function on value of ImmutableListMultimap instead of Map

I currently have this function:

ImmutableMap<String, ImmutableList<ImpactSummary>> toSummariesByKey(ImmutableMap<String, ImmutableList<Impact>> impactsByKey) return impactsByKey.entrySet().stream() .collect( toImmutableMap( entry -> entry.getKey(), entry -> helper(entry.getValue())));} 

which makes use of this function:

 ImmutableList<TierSummary> helper( ImmutableList<Impact> impacts) {...} 

As you can see, toSummariesByKey simply calls helper on each of the values in the input map.

Now, I need to modify toSummariesByKey to take as argument, and return, an ImmutableListMultimap instead:

 ImmutableListMultimap<String, ImpactSummary> toSummariesByKey( ImmutableListMultimap<String, Impact> impactsByKey) {...} 

Reusing the body as is causes, when the helper function is called, the following error:

type ImmutableList<Impact> is not compatible with Impact

However, helper is used in several places so I am reluctant to modify it. What is good practice in this scenario, please?