public override DownloadEndEventArgs? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
 {
-    return reader.GetDiscriminator("status") switch
+    var discriminator = reader.GetDiscriminator("status");
+    return discriminator switch
     {
         "canceled" => JsonSerializer.Deserialize(ref reader, options.GetTypeInfo<DownloadCanceledEventArgs>()),
         "complete" => JsonSerializer.Deserialize(ref reader, options.GetTypeInfo<DownloadCompleteEventArgs>()),
-        _ => null,
+        _ => throw new JsonException($"Unknown 'status' discriminator '{discriminator}' for DownloadEndEventArgs."),
     };
 }