Language-tools: Espera entonces sin variable

Describe el error
Recibo el error Cannot read property 'end' of null cuando uso {: then} sin una variable.

Reproducir

<script>
  let promise;
</script>

{#await promise}
  Loading...
{:then}
  Loaded!
{/await}

Comportamiento esperado
No hay error.

Capturas de pantalla

Contexto adicional
Producción:

Error walking node  {
  start: 35,
  end: 83,
  type: 'AwaitBlock',
  expression: Node {
    type: 'Identifier',
    start: 43,
    end: 50,
    loc: SourceLocation { start: [Position], end: [Position] },
    name: 'promise'
  },
  value: null,
  error: null,
  pending: {
    start: 51,
    end: 61,
    type: 'PendingBlock',
    children: [ [Object] ],
    skip: false
  },
  then: {
    start: 61,
    end: 75,
    type: 'ThenBlock',
    children: [ [Object] ],
    skip: false
  },
  catch: {
    start: null,
    end: null,
    type: 'CatchBlock',
    children: [],
    skip: true
  }
}