Microsoft.SharePoint.SPFieldUserValueCollection is not marked as serializable

Problem:
Develop on SharePoint Workflow
Unexpected    System.Workflow.Runtime.Hosting.PersistenceException: Type 'Microsoft.SharePoint.SPFieldUserValueCollection' in Assembly 'Microsoft.SharePoint, Version=12.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c' is not marked as serializable.
Snippet code causes error:

public sealed partial class YourWorkflow : SharePointSequentialWorkflowActivity

    {

        SPFieldUserValueCollection _reviewers;

Resolve:
Check fields in your custom activity or workflow, if it has field type Microsoft.SharePoint.SPFieldUserValueCollection, just remove it or use it as variable method.