Assigning Css Class To Masterpage Control From Contentpage In Asp

I have a unordered list in my master page....

<ul id="mainMenu" runat="server">
<li id="mainHome" runat="server"><a href="#" title="Home" class="home">
<span></span>Home</a></li>
<li id="mainManage" runat="server"><a href="Users.aspx" title="Manage"
class="manage"><span></span>Manage</a></li>
<li id="mainEnquiry" runat="server"><a href="account.aspx" title="Enquiry"
class="enquiry"><span></span>Enquiry</a></li>
<li id="mainReport" runat="server"><a href="EnquiryReport.aspx" title="Report"
class="report"><span></span>Report</a></li>
</ul>

From a content page i am assigning a css class to one of the list item.

HtmlGenericControl home = (HtmlGenericControl)this.Page.Master.FindControl("mainMenu").FindControl("mainManage") as HtmlGenericControl;
string cssToApply = "current";
if (null != home)
{
if (home.Attributes.ContainsKey("class"))
{
if (!home.Attributes["class"].Contains(cssToApply))
{
home.Attributes["class"] += " " + cssToApply;
}
}
else
{
home.Attributes.Add("class", cssToApply);
}
}
and my css,
#header ul li {
display:inline;
float:left;
}
#header ul a {
-x-system-font:none;
color:#FFFFFF;
display:block;
font-family:Trebuchet MS,Arial,sans-serif;
font-size:1.1em;
font-style:normal;
font-variant:normal;
font-weight:bold;
text-transform:uppercase;
text-decoration:none;
}
#header ul a {
-moz-border-radius:3px;
-webkit-border-radius:0.2em;
padding:3px 7px;
text-decoration:none;
}
#header ul a:focus, #header ul a:active, #header ul a:hover {
background-color:#829E7E;
outline-color:-moz-use-text-color;
outline-style:none;
outline-width:medium;
}
#header ul a.home {
margin:0 16px 0 17px;
}
#header ul #current a, #headermenu #current span{ /*currently selected tab*/
background-color: #BCE27F;
color:#666666;
white-space:nowrap;
}
#header ul a.manage,#header ul a.enquiry,#header ul a.report {
margin:0 14px 0 0;
}
#home #header ul a.home, #enquiry #header ul a.enquiry, #report #header ul a.report, #manage #header ul a.manage{
-moz-border-radius:3px;
-webkit-border-radius:0.2em;
background-color:#B9E27F;
color:#FFFFFF;
}

But i get the error, System.Web.UI.AttributeCollection' does not contain a definition for 'ContainsKey' and no extension method 'ContainsKey' accepting a first argument of type 'System.Web.UI.AttributeCollection' could be found (are you missing a using directive or an assembly reference?