Sharepoint: Value does not fall within the expected range - sharepoint

Question

I am trying to import a logo of a web and its subsite in sharepoint 2013.
When I try to load the file for subsite then exception occur.
Value does not fall within the expected range
Here is my code
if (siteLogo != null)
{
File = web.GetFileByServerRelativeUrl(siteLogo);
Data = File.OpenBinaryStream();
customContext.Context.Load(File);
customContext.Context.ExecuteQuery();
}
First Time it works fine. But when it loads for subsite it generate exception.
I tried alot to solve this but unable to find the solution. Please help.
This is complete file
public Branding(SharePointClientContext customContext, string siteLogo, string alternateCss, ListItemCollection ltItemCollection, Microsoft.SharePoint.Client.Web web)
{
if (ltItemCollection != null)
{
ListItem item = ltItemCollection.Where(a => a.DisplayName == "Current").FirstOrDefault();
if (item.FieldValuesAsText != null && item.FieldValuesAsText.FieldValues["ThemeUrl"] != null && item.FieldValuesAsText.FieldValues["ThemeUrl"] != "")
{
string themeUrl = item.FieldValuesAsText.FieldValues["ThemeUrl"].Split(',')[1];
string themeFileName = themeUrl.Substring(themeUrl.LastIndexOf('/') + 1).ToLower();
ListItem selectedTheme = ltItemCollection.Where(a =>
a.FieldValuesAsText.FieldValues["ThemeUrl"].Split(',')[1].Substring(a.FieldValuesAsText.FieldValues["ThemeUrl"].Split(',')[1].LastIndexOf('/') + 1) == themeFileName
&& a.DisplayName != "Current").FirstOrDefault();
this.Theme = selectedTheme.DisplayName;
}
}
if (siteLogo != null)
{
File = web.GetFileByServerRelativeUrl(siteLogo);
Data = File.OpenBinaryStream();
customContext.Context.Load(File);
customContext.Context.ExecuteQuery();
File = null;
}
if (alternateCss != null && alternateCss != "")
{
CssFile = web.GetFileByServerRelativeUrl(web.SiteLogoUrl);
CssData = CssFile.OpenBinaryStream();
web.Context.Load(CssFile);
web.Context.ExecuteQuery();
CssFile = null;
}
}
And calling this in following way.
SharePointConstants.branding.Add(new Branding(customContext, siteLogo, AlternateCss, ltItemCollection, web));