diff --git a/src/livecodes/languages/dart/lang-dart-runtime.ts b/src/livecodes/languages/dart/lang-dart-runtime.ts
index 234599e4b..4c2f0b2bd 100644
--- a/src/livecodes/languages/dart/lang-dart-runtime.ts
+++ b/src/livecodes/languages/dart/lang-dart-runtime.ts
@@ -3,7 +3,34 @@ import { dartWasmBaseUrl } from '../../vendors';
 declare const window: any;
 declare const parent: any;
 
+// The result page is served without a UTF-8 charset, so the browser falls back to its default
+// (windows-1252). The Dart/Flutter SDK loads its large, non-ASCII runtime scripts as
+// `text/javascript` blobs, and a blob script with no charset inherits the document's encoding,
+// which corrupts the SDK's data tables — an `Uri.parse` then throws
+// `RangeError (index): Index out of range`. Tagging those JS blobs with UTF-8 keeps the SDK
+// decoding correctly however the page is served. Both engines load through the same blob path.
+const forceUtf8ScriptBlobs = () => {
+  const scope = globalThis as any;
+  const OriginalBlob = scope.Blob;
+  if (typeof OriginalBlob !== 'function' || OriginalBlob.livecodesUtf8) return;
+
+  const Utf8Blob = function (parts: any, options: any) {
+    if (
+      options &&
+      typeof options.type === 'string' &&
+      /^(text|application)\/(java|ecma)script$/.test(options.type)
+    ) {
+      options = { ...options, type: `${options.type};charset=utf-8` };
+    }
+    return new OriginalBlob(parts, options);
+  };
+  Utf8Blob.prototype = OriginalBlob.prototype;
+  Utf8Blob.livecodesUtf8 = true;
+  scope.Blob = Utf8Blob;
+};
+
 export const setupDartRuntime = (engine: 'dart' | 'flutter') => {
+  forceUtf8ScriptBlobs();
   window.livecodes = window.livecodes || {};
   const ns = (window.livecodes[engine] = window.livecodes[engine] || {});