ZipArchive CreateEntry returns null for valid filepath

I am having difficulty with the ZipArchive's CreateEntry function. On Android, entry is defined and the functions completes. However on iOS, the entry I get back is null. I have tested the filename to make sure that it is accurate. Does anyone know why I am getting these results and how to fix it?

public static async Task<ZipArchiveEntry> AddFileAsync(string filename)
 {
            using (var memoryStream = new MemoryStream())
            using (archive = new ZipArchive(memoryStream, ZipArchiveMode.Create, true))
            {
                var entry = archive.CreateEntry(filename);
                var zipStream = entry.Open();
        ...