Value cannot be null. Parameter name: key, Source: mscorlib

I am getting this error, and not enough info in stack to help me. 
I have seen other posts similar to this error, but none of the solutions were applicable.
Help is most appreciated.
Here is the info I have:
Message: Value cannot be null. Parameter name: key
Source: mscorlib
Method: Int32 FindEntry(TKey)
Stack Trace:    at System.Collections.Generic.Dictionary`2.FindEntry(TKey key)
   at AjaxControlToolkit.ToolkitScriptManagerCombiner.GetCombinedScriptContent(HttpContextBase context, String[] bundles, String hash)   
   at AjaxControlToolkit.ToolkitScriptManagerCombiner.OutputCombinedScriptFile(HttpContextBase context)   
   at AjaxControlToolkit.ToolkitScriptManager.OnInit(EventArgs e)   
   at System.Web.UI.Control.InitRecursive(Control namingContainer)   
   at System.We ...