sharepoint workflow assigned to field is empty

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SharePoint

I am making a sharepoint state machine workflow on a document library
in my library i have people picker field "reviewers" that may contain a values like this

reviewer1;reviewer2; reviewer3;

now in my workflow on createtask event i am writing something as below

createTaskForQD.TaskId = Guid.NewGuid();
createTaskForQD_TaskProperties1.Title = "Review Document";
createTaskForQD_TaskProperties1.AssignedTo = workflowProperties.Item["reviewers"].ToString();


after running when i check in the task list the "AssignedTo" field is empty?
what should i do?? help