How to override effect handler with a different type argument?

[chtenb]

import std/core/list

pub effect yield<a>
  ctl yield(elem : a) : ()

// You can override handlers with the override keyword. This uninstalls the original
// handler and forwards any effect operation calls to the new effect.
  
fun double(inner : () -> <yield<a>|e> ()) : <yield<a>|e> ()
  with override ctl yield(elem)
    yield(elem) // Forward the yield to our new yield operation
    yield(elem) // Forward the yield to our new yield operation
  inner()

// But it doesn't seem to work when the type arguments are different
fun flatten(yield-lists : () -> <yield<list<a>>|e> ()) : <yield<a>|e> ()
  with override ctl yield(elems)
    elems.list/foreach fn(elem)
      yield(elem) // Forward the yield to our new yield operation
  yield-lists()

Error:

repro.kk(18, 3): type error: types do not match (due to an infinite type)
  context        :   with override ctl yield(elems)
                       elems.list/foreach fn(elem)
                         yield(elem)
  term           :   with override ctl yield(elems)
                       elems.list/foreach fn(elem)
                         yield(elem)
  inferred effect: <yield<list<_a>>,yield<list<_a>>|_e1>
  expected effect: <yield<list<_a>>,yield<_a>|_e>
  hint           : give a type to the function definition?