ASP.NET Session problem

[10 weber 0]

I wrote in a web masterpage

<div style="display:inline-block; background-image:url(Images/star.svg); width:<%=((int)Session["adminLevel"] * 31) %>px; height:30px;"></div><div style="display:inline-block; background-image:url(Images/emptyStar.svg); width:<%=(93 - (int)Session["adminLevel"] * 31) %>px; height:30px;"></div>

to show users their adminLevel parameter. It's supposed to show a bar of 3 stars, some of which are full and the other empty. To do that I wrote

Server Error in '/' Application.Object reference not set to an instance of an object.Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.Source Error:Line 25:                 %>Line 26:                 <br />Line 27:                 <div style="display:inline-block; background-image:url(Images/star.svg); width:<%=((int)Session["adminLevel"] * 31) %>px; height:30px;"></div>Line 28:                 <div style="display:inline-block; background-image:url(Images/emptyStar.svg); width:<%=(93 - (int)Session["adminLevel"] * 31) %>px; height:30px;"></div>Line 29:              </td>Source File: c:\Users\ADMIN\Documents\Visual Studio 2010\Projects\web\web\Site.Master    Line: 27Stack Trace:[NullReferenceException: Object reference not set to an instance of an object.]   ASP.site_master.__Render__control1(HtmlTextWriter __w, Control parameterContainer) in c:\Users\ADMIN\Documents\Visual Studio 2010\Projects\web\web\Site.Master:27   System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +109   System.Web.UI.Control.RenderChildren(HtmlTextWriter writer) +8   System.Web.UI.Control.Render(HtmlTextWriter writer) +10   System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) +27   System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter) +100   System.Web.UI.Control.RenderControl(HtmlTextWriter writer) +25   System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +208   System.Web.UI.Control.RenderChildren(HtmlTextWriter writer) +8   System.Web.UI.Page.Render(HtmlTextWriter writer) +29   System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) +27   System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter) +100   System.Web.UI.Control.RenderControl(HtmlTextWriter writer) +25   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +3060Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.0.30319.1