diff --git a/.changeset/utf8-last-event-id.md b/.changeset/utf8-last-event-id.md
new file mode 100644
index 0000000..7dfce9f
--- /dev/null
+++ b/.changeset/utf8-last-event-id.md
@@ -0,0 +1,7 @@
+---
+'eventsource': patch
+---
+
+Fixed `Last-Event-ID` not being sent as UTF-8 on reconnect
+
+The event ID was passed to `fetch()` as a plain string header value, but header values are byte strings. An ID containing a character above U+00FF, eg `€`, made `fetch()` throw a `TypeError` on every reconnect attempt, so the connection was never re-established. An ID with characters in the U+0080 to U+00FF range, eg `é`, was sent as a single Latin-1 byte instead of its UTF-8 encoding. The header now carries the UTF-8 bytes of the ID, as the specification requires.
diff --git a/src/EventSource.ts b/src/EventSource.ts
index e4dd9c6..ec24ada 100644
--- a/src/EventSource.ts
+++ b/src/EventSource.ts
@@ -559,7 +559,11 @@ class EventSourceImpl extends EventTarget implements EventSource {
    * @internal
    */
   #getRequestOptions(): EventSourceFetchInit {
-    const lastEvent = this.#lastEventId ? {'Last-Event-ID': this.#lastEventId} : undefined
+    // [spec] If the last event ID string is not the empty string, set `Last-Event-ID` to the last
+    // event ID string, encoded as UTF-8.
+    const lastEvent = this.#lastEventId
+      ? {'Last-Event-ID': utf8ToByteString(this.#lastEventId)}
+      : undefined
 
     const init: EventSourceFetchInit = {
       // [spec] Let `corsAttributeState` be `Anonymous`…
@@ -810,3 +814,24 @@ function defineEventProperty(
     configurable: true,
   })
 }
+
+/**
+ * Encodes a string as UTF-8 and returns those bytes as a "byte string": one character per byte,
+ * each in the range U+0000 to U+00FF.
+ *
+ * Header values are byte strings. `fetch()` throws a `TypeError` for any character above U+00FF,
+ * and sends a character in U+0080 to U+00FF as a single Latin-1 byte, rather than as its UTF-8
+ * encoding. Passing the UTF-8 bytes through as characters makes `fetch()` put exactly those
+ * bytes on the wire.
+ *
+ * @param value - The string to encode
+ * @returns The UTF-8 bytes of `value`, one per character
+ * @internal
+ */
+function utf8ToByteString(value: string): string {
+  let byteString = ''
+  for (const byte of new TextEncoder().encode(value)) {
+    byteString += String.fromCharCode(byte)
+  }
+  return byteString
+}
diff --git a/test/client.test.ts b/test/client.test.ts
index d807576..595c30a 100644
--- a/test/client.test.ts
+++ b/test/client.test.ts
@@ -480,6 +480,33 @@ test('an ID from a block without data is used for reconnecting and later message
   await deferClose(es)
 })
 
+const nonAsciiIds = [
+  {name: 'Latin-1 (U+0080 to U+00FF)', id: 'café'},
+  {name: 'beyond Latin-1 (above U+00FF)', id: 'café-€1'},
+  {name: 'outside the BMP', id: 'id-😀'},
+]
+
+test.each(nonAsciiIds)(
+  'sends a UTF-8 encoded `Last-Event-ID` for an ID with characters $name',
+  async ({id}) => {
+    const onMessage = getCallCounter({name: 'onMessage'})
+    const url = `${serverUrl}/echo-last-event-id?id=${encodeURIComponent(id)}`
+    const es = new OurEventSource(url, esInit)
+
+    es.addEventListener('message', onMessage.listener)
+
+    // The first message arrives together with the ID, the second is the echo from the reconnect
+    await onMessage.waitForCallCount(2)
+
+    expect(onMessage.lastArg).toMatchObject({
+      data: `Reconnected with ${id}`,
+      lastEventId: id,
+    })
+
+    await deferClose(es)
+  },
+)
+
 test('will not reconnect after explicit `close()`', async () => {
   const onMessage = getCallCounter({name: 'onMessage'})
   const onError = getCallCounter<ErrorEvent>({name: 'onError'})
diff --git a/test/helpers/server.ts b/test/helpers/server.ts
index ace2733..a8e0646 100644
--- a/test/helpers/server.ts
+++ b/test/helpers/server.ts
@@ -65,6 +65,8 @@ export function handleRequest(
       return writeMessageBurst(req, res)
     case '/id-only':
       return writeIdOnly(req, res)
+    case '/echo-last-event-id':
+      return writeEchoLastEventId(req, res)
     case '/identified':
       return writeIdentifiedListeners(req, res)
     case '/end-after-one':
@@ -192,6 +194,30 @@ function writeIdOnly(req: IncomingMessage, res: ServerResponse) {
   res.end()
 }
 
+/**
+ * Sends the event ID given in the `id` query parameter on the first connection, then closes. On the
+ * reconnect, echoes back the `Last-Event-ID` header, decoded as UTF-8, so that a test can check
+ * exactly which bytes the client put on the wire.
+ */
+function writeEchoLastEventId(req: IncomingMessage, res: ServerResponse) {
+  res.writeHead(200, {
+    'Content-Type': 'text/event-stream',
+    'Cache-Control': 'no-cache',
+    Connection: 'keep-alive',
+  })
+
+  const lastEventId = getLastEventIdFromUtf8Header(req)
+  if (lastEventId === undefined) {
+    const id = new URL(req.url || '/', 'http://localhost').searchParams.get('id') ?? ''
+    tryWrite(res, encode({retry: 50, id, data: 'First connection'}))
+    res.end()
+    return
+  }
+
+  tryWrite(res, encode({data: `Reconnected with ${lastEventId}`}))
+  res.end()
+}
+
 async function writeIdentifiedListeners(req: IncomingMessage, res: ServerResponse) {
   const url = new URL(req.url || '/', 'http://localhost')
   const clientId = url.searchParams.get('client-id')
@@ -601,6 +627,15 @@ function getLastEventId(req: IncomingMessage): string | undefined {
   return typeof lastId === 'string' ? lastId : undefined
 }
 
+/**
+ * Like `getLastEventId()`, but for a header value that carries UTF-8 bytes. Node hands header
+ * values over with one character per byte, so the bytes have to be decoded explicitly.
+ */
+function getLastEventIdFromUtf8Header(req: IncomingMessage): string | undefined {
+  const lastId = getLastEventId(req)
+  return lastId === undefined ? undefined : Buffer.from(lastId, 'latin1').toString('utf8')
+}
+
 function tryWrite(res: ServerResponse, chunk: string | Uint8Array) {
   try {
     res.write(chunk)