Add the server variable name to the allowed server variable list.

In IIS you can add Server variable to the server variable list in the following steps.

1. Go to IIS
2. Select the site node for which you want to add variable
3. double click URL Rewrite under IIS on the right hand side. If you do not see it you may want to install it from Microsoft site.
4. On the right column, select “View Server Variables”.
5. If you have any variables you will see it, if you don’t and want to add one, select Add button in the top right column
6. Type name name of the variable in the popup dialog and you are all set.