iframe parser error (HtmlIframe/HtmlGenericControl) but still using .NET 4.0 (not .NET 4.5)

Another person had the same issue as I did and it was resolved in this post: https://stackoverflow.com/a/21707234/156611

Basically, make sure your compilation targetFramework is set to 4.6.1 (or whatever you are using) and then go into your ASPX/ASCX file throwing the error and save it. The saving will re-create the designer file with the appropriate type.

in my case, it changed from:

System.Web.UI.HtmlControls.HtmlGenericControl

to:

System.Web.UI.HtmlControls.HtmlIframe