Assigning Person/Group column type value from one list to another list.

i am assigning value to a user column from a user column in a  list to another list using the following code

item["Username"] = CurrentListItem["Username"].ToString();

The code works fine when there is a value in CurrentListItem.But when there is no value in CurrentListItem it throws an null value exception.Any pointers on how to handle this will be helpful.