How to force my lambda expressions to evaluate early? Fix lambda expression weirdness?

I have written the following C# code:

_locationsByRegion = new Dictionary<string, IEnumerable<string>>();
foreach (string regionId in regionIds)
{
    IEnumerable<string> locationIds = Locations
        .Where(location => location.regionId.ToUpper() == regionId.ToUpper())
        .Select(location => location.LocationId); //If I cast to an array here, it works.
    _locationsByRegion.Add(regionId, LocationIdsIds);
}

This code is meant to create a a dictionary with my "region ids" as keys and lists of "location ids" as values.

However, what actually happens is that I get a dictionary with the "region ids" as keys, but the value for each key is identical: it is the list of locations for the last region id in regionIds!

It looks like this is a product of how lambda expressions are evaluated. I can get the correct result by casting the list of location ids to an array, but this feels like a kludge.

What is a good practice for handling this situation?