diff --git a/json/concatenated_json_parse_stream.ts b/json/concatenated_json_parse_stream.ts
index 3d41fba574f2..5437476244bb 100644
--- a/json/concatenated_json_parse_stream.ts
+++ b/json/concatenated_json_parse_stream.ts
@@ -3,9 +3,8 @@ import { toTransformStream } from "../streams/to_transform_stream.ts";
 import type { JsonValue, ParseStreamOptions } from "./common.ts";
 import { parse } from "./_common.ts";
 
-const blank = new Set(" \t\r\n");
-function isBlankChar(char: string) {
-  return blank.has(char);
+function isBlankChar(char: string | undefined) {
+  return char !== undefined && [" ", "\t", "\r", "\n"].includes(char);
 }
 
 const primitives = new Map(