Could not load type 'System.Web.UI.ScriptReferenceBase' from assembly 'System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf385

Ajax Error :
Could not load type 'System.Web.UI.ScriptReferenceBase' from assembly 'System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf385
I got this error while trying to use the Calendar control extender from the Ajax Control Toolkit for one of my textboxes.
First I got an error "sys.extended.ui is null or not an object" . To fix this, I changed the ScriptManager control to use the "ToolkitScriptManager" .
Once I did that I started getting this new error:Could not load type 'System.Web.UI.ScriptReferenceBase' from assembly 'System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf385
This got resolved once I installled .Net Framework 3.5 SP1.
That got my Ajax calendar control working like a charm ;)