How can a C# XSLT extension function return an XML fragment that gets used as unescaped XML?

I've written an XSLT which uses a C# extension function. That function returns an XML fragment. Something like this:

public string MyFunc(string input)
{
  return "<thisIsXml/>";
}

I call it like this:

<value-of select="extensions:MyFunc(.)"/>

It works, but the response is escaped, so it ends up in the result like this:

<thisIsXml>

I don't want that -- I want this injected as actual, unescaped XML. Possible?

(This is C#, so XSLT 1.0 only.)