主题：val字符转换

        Dim dr1 As DataRow = DataTables("初始号").AddNew()
        dr1("收购凭证起始号") = vars("fp1")
        dr1("收购凭证终止号") = vars("fp2")
        Dim s1 As String = dr1("收购凭证起始号")
        Dim s2 As String =  dr1("收购凭证终止号")
        dr("收购piao数量") = val(s2)- val(s1)

运行错误：Object reference not set to an instance of an object.

为什么用val转换字符为数值运算出错？