ArgumentOutOfRangeException: StartIndex 不能小于 0

[ArgumentOutOfRangeException: StartIndex 不能小于 0。
参数名: startIndex]
   System.String.InternalSubStringWithChecks(Int32 startIndex, Int32 length, Boolean fAlwaysCopy) +7500031
   XXGL_ContentEdit.GetImgName() +133
   XXGL_ContentEdit.btn_Mod_Click(Object sender, EventArgs e) +235
   System.Web.UI.WebControls.Button.OnClick(EventArgs e) +111
   System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument) +110
   System.Web.UI.WebControls.Button.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument) +10
   System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument) +13
   System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData) +36
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +1565