diff --git a/index.bs b/index.bs
index 8b9bf7ad2..03774cb42 100644
--- a/index.bs
+++ b/index.bs
@@ -5373,6 +5373,10 @@ The following abstract operations support the implementation of the
  id="writable-stream-default-controller-get-chunk-size">WritableStreamDefaultControllerGetChunkSize(|controller|,
  |chunk|)</dfn> performs the following steps:
 
+ 1. If |controller|.[=WritableStreamDefaultController/[[strategySizeAlgorithm]]=] is undefined, then:
+  1. Assert: |controller|.[=WritableStreamDefaultController/[[stream]]=].[=WritableStream/[[state]]=] is "`erroring`" or
+     "`errored`".
+  1. Return 1.
  1. Let |returnValue| be the result of performing
     |controller|.[=WritableStreamDefaultController/[[strategySizeAlgorithm]]=], passing in |chunk|,
     and interpreting the result as a [=completion record=].
diff --git a/reference-implementation/lib/abstract-ops/writable-streams.js b/reference-implementation/lib/abstract-ops/writable-streams.js
index cf303bfe7..02b2ee7b1 100644
--- a/reference-implementation/lib/abstract-ops/writable-streams.js
+++ b/reference-implementation/lib/abstract-ops/writable-streams.js
@@ -662,6 +662,11 @@ function WritableStreamDefaultControllerGetBackpressure(controller) {
 }
 
 function WritableStreamDefaultControllerGetChunkSize(controller, chunk) {
+  if (controller._strategySizeAlgorithm === undefined) {
+    assert(controller._stream._state === 'erroring' || controller._stream._state === 'errored');
+    return 1;
+  }
+
   try {
     return controller._strategySizeAlgorithm(chunk);
   } catch (chunkSizeE) {