I get "Input string was not in a correct format." on the following when attempting to pass a null value:

ProductType = new clsProductType();

ProductType.SortOrder = Int32.Parse(txtSortOrder.Text); <----- This Line.
//ProductType.SortOrder = Convert.ToInt32(txtSortOrder.Text); <----- Also tried this

Sort Order is declared as such:

public int? SortOrder { get; set; }

In the constructor for the class i have SortOrder = null, and elsewhere in the code have it take a number from a textbox

SortOrder is used to pass a variable so that it can be written to an SQL Table. The value needs to accept an integer but can also be null-able.

Any Ideas?