diff --git a/flasher/src/main.ts b/flasher/src/main.ts
index 02fd36ea0..e3e594ae8 100644
--- a/flasher/src/main.ts
+++ b/flasher/src/main.ts
@@ -117,7 +117,22 @@ const terminal = {
 function post(msg: OutboundMessage): void {
   // targetOrigin narrows from '*' to the opener's real origin once known; see
   // its declaration. Outbound frames carry no nonce (see protocol.ts).
-  opener?.postMessage(msg, targetOrigin);
+  try {
+    opener?.postMessage(msg, targetOrigin);
+  } catch (err) {
+    // A malformed origin= hash param (e.g. origin=null) makes postMessage throw,
+    // which would wedge the ready handshake. Fall back to '*' so frames keep
+    // flowing; outbound frames carry no nonce (see protocol.ts), so the broader
+    // audience leaks nothing. Log rather than swallow so an unrelated failure
+    // (not just a bad origin) is still visible.
+    console.error("Flasher postMessage failed; falling back to '*':", err);
+    targetOrigin = "*";
+    try {
+      opener?.postMessage(msg, "*");
+    } catch (err2) {
+      console.error("Flasher postMessage failed after origin fallback:", err2);
+    }
+  }
 }
 
 function setState(state: FlashState, detail: string): void {
@@ -167,6 +182,9 @@ window.addEventListener("message", (ev: MessageEvent) => {
   if (!data || data.type !== "esphome-web-flash:firmware") return;
   if (data.nonce !== nonce) return;
   if (!isFlashParts(data.parts)) {
+    // The opener has attached and sent, so stop re-announcing ready even though
+    // the payload is unusable, mirroring the accepted path below.
+    stopReadyRetry();
     setState("error", "Received a malformed firmware payload.");
     return;
   }
@@ -504,7 +522,7 @@ installBtn.addEventListener("click", async () => {
       data: new Uint8Array(p.data),
       address: p.address,
     }));
-    await runFlash(files, firmware.erase ?? true);
+    await runFlash(files, firmware.erase !== false);
     return;
   }
   const file = fileInput.files?.[0];
diff --git a/flasher/test/handshake.test.mjs b/flasher/test/handshake.test.mjs
index d86d3df3c..56b8725c5 100644
--- a/flasher/test/handshake.test.mjs
+++ b/flasher/test/handshake.test.mjs
@@ -114,6 +114,18 @@ try {
   else if (enabledAfterBad) fail("install enabled after malformed payload");
   else console.log("PASS: malformed payload rejected with error state");
 
+  // 2b-ii. ...and the malformed payload stops the ready re-announce: the opener
+  // has clearly attached, so it must not keep receiving ready frames.
+  await a.evaluate(() => {
+    window.__msgs.length = 0;
+  });
+  await new Promise((r) => setTimeout(r, 700));
+  const readyAfterBad = (await a.evaluate(() => window.__msgs)).some(
+    (m) => m && m.type === "esphome-web-flash:ready",
+  );
+  if (readyAfterBad) fail("ready still re-announced after malformed payload");
+  else console.log("PASS: ready retry stopped after malformed payload");
+
   // 2c. a negative/non-integer address is rejected at the boundary
   await a.evaluate(() => {
     window.__b.postMessage(
@@ -132,6 +144,26 @@ try {
     fail("negative address not rejected: " + label);
   else console.log("PASS: negative address rejected at boundary");
 
+  // 2d. a malformed origin= hash param must not wedge the ready handshake:
+  // postMessage to a bad targetOrigin throws, and the receiver falls back to '*'.
+  {
+    const c = await browser.newPage();
+    await c.goto(`${base}/opener.html`);
+    const [pop2] = await Promise.all([
+      new Promise((res) => c.once("popup", res)),
+      c.evaluate((u) => window.__open(u), `${base}/#nonce=n2&origin=null`),
+    ]);
+    await pop2.waitForNetworkIdle({ idleTime: 300 }).catch(() => {});
+    await new Promise((r) => setTimeout(r, 300));
+    const ready2 = (await c.evaluate(() => window.__msgs)).find(
+      (m) => m && m.type === "esphome-web-flash:ready",
+    );
+    if (!ready2) fail("ready not received when origin= hash param is malformed");
+    else console.log("PASS: malformed origin falls back to '*', ready still sent");
+    await pop2.close();
+    await c.close();
+  }
+
   // 3. correct nonce accepted -> button enabled, state mirrored back
   await a.evaluate(() => {
     window.__b.postMessage(