signal handler argument is erroneously undefined

signal save(MyValue value)

// forwarding a signal with parameter
onInnerSave: (value) => { 
  console.log(value)          // prints "undefined"
  root.save(value)            // calls signal which fails due to undefined
} 

signal save(MyValue value)

function saveWrapped(value) { 
  console.log(value)          // prints the correct object 
  root.save(value)            // calls signal, works just fine
} 

onInnerSave: (value) => { 
  saveWrapped(value) 
}  

signal innerSave(MyValue value)