access 导入 access 转换无效!在线等!

从类型"DBNULL" 到类型“Double”的转换无效（数据库中对应的 a3 或a5 没有数据是为空）
,请问我怎么处理！

VB.NET code:

Dim a1 As String = Nothing
Dim a2 As String = Nothing
Dim a3 As Integer
Dim a4 As String = Nothing

Dim a5 As Double
Dim a6 As String = Nothing
Dim a7 As String = Nothing

Dim SQL As String = Nothing
For s As Integer = 0 To dsin.Tables("kscjin").Rows.Count - 1
a1 = dsin.Tables("kscjin").Rows(s).Item("kh") & ""
a2 = dsin.Tables("kscjin").Rows(s).Item("xbdm") & ""
a3 = dsin.Tables("kscjin").Rows(s).Item("xh")
a4 = dsin.Tables("kscjin").Rows(s).Item("cj") & ""
a5 = dsin.Tables("kscjin").Rows(s).Item("df")
a6 = dsin.Tables("kscjin").Rows(s).Item("zh") & ""
a7 = dsin.Tables("kscjin").Rows(s).Item("sxh") & ""
SQL = "insert into kscj(kh,