@@ -1167,9 +1167,18 @@ function writableStreamDefaultControllerGetDesiredSize(controller) {
 }
 
 function writableStreamDefaultControllerGetChunkSize(controller, chunk) {
+const {
+stream,
+sizeAlgorithm,
+} = controller[kState];
+if (sizeAlgorithm === undefined) {
+assert(stream[kState].state === 'errored' || stream[kState].state === 'erroring');
+return 1;
+}
+
 try {
 return FunctionCall(
-controller[kState].sizeAlgorithm,
+sizeAlgorithm,
 undefined,
 chunk);
 } catch (error) {