Value of type 'String' cannot be converted to '1-dimensional array of String'

I want to pass string values from my listbox to a function which is located in another file, I get an error when i try to pass values in
AddUsersToRoles()

Please Advice:

        Dim lstItem As ListItem
        For Each SelectedItem In CType(SelectRoles.FindControl("AvailableRoles"), ListBox).Items
            If lstItem .Selected = True Then
                MyRole= lstItem .Value
                System.Web.Security.Roles.AddUsersToRoles(CreateUserWizard1.UserName, MyRole)
            End If
        Next

Error: Value of type 'String' cannot be converted to '1-dimensional array of String'
this errors is in: AddUsersToRoles(CreateUserWizard1.UserName, MyRole)

AddUsersToRoles Object Browswer:
Public Shared Sub AddUsersToRoles(ByVal usernames() As String, ByVal roleNames() As String)
     Member of: System.Web.Security.Roles
Summary:
Adds the specified users to the specified roles.

Parameters:
roleNames: A string array of role names to add the specified user names to.
usernames: A string array of user names to add to the specified roles.

Exceptions:
System.Configuration.Provider.ProviderException: Role management is not enabled.
System.ArgumentNullException: One of the roles in roleNames is null.-or-One of the users in usernames is null.
System.ArgumentException: One of the roles in roleNames is an empty string or contains a comma (,).-or-One of the users in usernames is an empty string or contains a comma (,).-or-roleNames contains a duplicate element.-or-usernames contains a duplicate element.