Index out of range after SQL execution

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VB

Hi, I have a inline query SELECT statement written in VB. When I try to run it, it prompt me an error message IndexOutOfRangeException. I tried to debug it and found the error prompt after ExecuteReader().
At first I thought it was due to my returned value is NULL but the error still persist after I have added IsDBNull in my codes.
Below are my codes:

Try
    cnSql = New SqlConnection(SQL_CONNECTION_STRING)
    cnSql.Open()

    strSQL = "SELECT ISNULL(MIN(SH.shipping_start_date), GETDATE()) AS StartDate "
    strSQL = strSQL + "FROM Shipping AS SH INNER JOIN Working AS WK ON SH.shipping_id = WK.shipping_id "
    strSQL = strSQL + "WHERE WK.woNo = '" + number + "'"

    cmSql = New SqlCommand(strSQL, cnSql)
    readerSQL = cmSql.ExecuteReader()

    If readerSQL.Read() Then
        If Not IsDBNull(readerSQL.Item("shipping_start_date")) Then
            woStartDate = readerSQL.Item("shipping_start_date")
        End If
    Else
        ShipStartDate = DateTime.Now
    End If


    readerSQL.Close()
    cnSql.Close()
    cmSql.Dispose()
    cnSql.Dispose()

Catch ex As SqlException
    MsgBox(ex.Message, MsgBoxStyle.Critical, "SQL Error!")

Catch ex As Exception
    MsgBox(ex.Message, MsgBoxStyle.Critical, "General Error!")'error happen here

What I have tried:

1. Added IsDBNull in my codes to filter out NULL value returned from DB.
2. Tried to excute the query directly in SQL Server without any error.