diff --git a/README.md b/README.md
index 35e0a95..975c2ce 100644
--- a/README.md
+++ b/README.md
@@ -2129,14 +2129,19 @@ view would leave a wallet one click from deletion.
   `wallet_requestPermissions` and is on neither the allowed nor the denied list.
   A site is its full origin, `scheme://host[:port]`, on both lists and for a
   connection allowed without "Remember": a choice for `https://dapp.example`
-  says nothing about `http://dapp.example` or another port of that host. The
-  background script prefers the toolbar popup (`action.openPopup()`) and falls
-  back to a separate popup window (`src/background/index.js`,
-  `requestApproval()`). Only one exists per site at a time: a further connection
-  request from a site whose prompt is still unanswered is refused with EIP-1193
-  code `-32002` and opens no new prompt. If that prompt was in a toolbar popup
-  that closed before it connected, and the toolbar popup has since been set to
-  open something else, the refused request shows that prompt again.
+  says nothing about `http://dapp.example` or another port of that host. A
+  request whose origin has no host names no site, so it is refused with EIP-1193
+  code `4100` and opens no prompt. Two such origins are `null`, which the
+  browser reports for a page with an opaque origin, such as one its server
+  sandboxes with a Content-Security-Policy, and `file://`, which Chrome reports
+  for a local file. The background script prefers the toolbar popup
+  (`action.openPopup()`) and falls back to a separate popup window
+  (`src/background/index.js`, `requestApproval()`). Only one exists per site at
+  a time: a further connection request from a site whose prompt is still
+  unanswered is refused with EIP-1193 code `-32002` and opens no new prompt. If
+  that prompt was in a toolbar popup that closed before it connected, and the
+  toolbar popup has since been set to open something else, the refused request
+  shows that prompt again.
 - **Elements**:
     - "Connection Request" heading
     - Phishing warning banner (shown when the hostname is on the phishing
diff --git a/src/background/index.js b/src/background/index.js
index a2bc512..8b3fe77 100644
--- a/src/background/index.js
+++ b/src/background/index.js
@@ -1457,7 +1457,14 @@ runtime.onMessage.addListener((msg, sender, sendResponse) => {
         // the message carries. Firefox before 126 gives no sender.origin, so
         // the origin of sender.url is used: the frame that sent the message,
         // not the tab's page, which may be another site embedding that
-        // frame. With neither, the request is refused.
+        // frame.
+        //
+        // A site is scheme://host[:port], so a request is refused when its
+        // origin has no host, as when it has no origin at all, rather than
+        // credited to the site its URL names. The browser reports "null" for
+        // a page with an opaque origin, such as one its server sandboxes with
+        // a Content-Security-Policy, and Chrome reports "file://" for a local
+        // file.
         let trustedOrigin = sender.origin;
         if (!trustedOrigin && sender.url) {
             try {
@@ -1466,7 +1473,13 @@ runtime.onMessage.addListener((msg, sender, sendResponse) => {
                 // an unparseable URL leaves the origin unknown
             }
         }
-        if (!trustedOrigin) {
+        let host = "";
+        try {
+            host = new URL(trustedOrigin).host;
+        } catch {
+            // no origin, or "null", is not a URL and so has no host
+        }
+        if (!host) {
             sendResponse({
                 error: {
                     code: 4100,
diff --git a/tests/rpcOrigin.test.js b/tests/rpcOrigin.test.js
index 108b885..e72c700 100644
--- a/tests/rpcOrigin.test.js
+++ b/tests/rpcOrigin.test.js
@@ -8,8 +8,15 @@
 // and a request the browser said nothing about was credited to whatever the
 // page wrote (https://git.eeqj.de/sneak/AutistMask/issues/407).
 //
-// Every sender here lacks sender.origin, as on old Firefox. The connection
-// check on eth_accounts is what shows which site a request was credited to.
+// An origin with no host names no site, so its request is refused
+// (https://git.eeqj.de/sneak/AutistMask/issues/527). The browser reports
+// "null" for a page with an opaque origin, such as one its server sandboxes
+// with a Content-Security-Policy, and Chrome reports "file://" for a local
+// file.
+//
+// Every other sender here lacks sender.origin, as on old Firefox. The
+// connection check on eth_accounts is what shows which site a request was
+// credited to.
 
 const { makeStorageStub } = require("./support/storageStub");
 
@@ -144,3 +151,41 @@ describe("a request the browser does not say the sender of", () => {
         });
     });
 });
+
+describe("a request whose origin has no host", () => {
+    test("from a page with an opaque origin is refused, whatever its URL says", async () => {
+        const accounts = loadBackground();
+
+        const result = await accounts({
+            origin: "null",
+            url: CONNECTED_ORIGIN + "/sandboxed.html",
+            tab: { url: CONNECTED_ORIGIN + "/sandboxed.html" },
+        });
+
+        expect(result).toEqual({
+            error: {
+                code: 4100,
+                message:
+                    "The wallet could not tell which site sent this request.",
+            },
+        });
+    });
+
+    test("from a local file is refused", async () => {
+        const accounts = loadBackground();
+
+        const result = await accounts({
+            origin: "file://",
+            url: "file:///home/user/page.html",
+            tab: { url: "file:///home/user/page.html" },
+        });
+
+        expect(result).toEqual({
+            error: {
+                code: 4100,
+                message:
+                    "The wallet could not tell which site sent this request.",
+            },
+        });
+    });
+});