Add an <assembly> to web.config with SPWebConfigModification

Question

I am trying to add an assembly to configuration/system.web/compilation/assemblies - but with the following code it I get Expression must evaluate to a node-set.

private SPWebConfigModification CreateWebModificationObject()
    {
        string strUniqueName = string.Format("add [@assembly='{0}']", typeof(HowsYourDayHub).AssemblyQualifiedName);
        string strNode = "configuration/system.web/compilation/assemblies";

        SPWebConfigModification webConfigModification = new SPWebConfigModification(strUniqueName, strNode)
            {
                Owner = "HowsYourDay",
                Sequence = 1,
                Type = SPWebConfigModification.SPWebConfigModificationType.EnsureChildNode,
                Value = string.Format("<add assembly='{0}' />", typeof(HowsYourDayHub).AssemblyQualifiedName)
            };

        return webConfigModification;
    }

Answer 1

Okay I found a solution with a console app - seems like a previous modification was stuck:

            SPWebApplication webapp = site.WebApplication;
            webapp.WebConfigModifications.Clear();

            webapp.Update();
            webapp.WebService.WebConfigModifications.Clear();
            webapp.WebService.ApplyWebConfigModifications();

after clearing every modification it started working!