Extension Methods for Inner Class?

Is it possible to define an extension method for an inner class? The syntax reference seems to show it is possible to have a UsingParamClause before the DefParam, which is promising, and motivates the following example:

class Foo:
  class Bar

extension (using foo: Foo)(bar: foo.Bar) def baz: String = "hi"

val myFoo: Foo = new Foo
val myBar: myFoo.Bar = new myFoo.Bar

val str: String = myBar.baz()

The last line gives me the following error:

value baz is not a member of myFoo.Bar.
An extension method was tried, but could not be fully constructed:

    baz(/* missing */summon[Foo])(myBar)

    failed with:

        No given instance of type Foo was found for parameter foo of method baz

I would expect that myFoo would be a suitable given instance of type Foo. Do I need to use a given instance?