diff --git a/lib/renderer/window-setup.js b/lib/renderer/window-setup.js
index 21f0741a22aa..d29133c88f0b 100644
--- a/lib/renderer/window-setup.js
+++ b/lib/renderer/window-setup.js
@@ -121,11 +121,11 @@ module.exports = (ipcRenderer, guestInstanceId, openerId, hiddenPage) => {
   }
 
   window.alert = function (message, title) {
-    ipcRenderer.sendSync('ELECTRON_BROWSER_WINDOW_ALERT', message, title)
+    ipcRenderer.sendSync('ELECTRON_BROWSER_WINDOW_ALERT', `${message}`, `${title}`)
   }
 
   window.confirm = function (message, title) {
-    return ipcRenderer.sendSync('ELECTRON_BROWSER_WINDOW_CONFIRM', message, title)
+    return ipcRenderer.sendSync('ELECTRON_BROWSER_WINDOW_CONFIRM', `${message}`, `${title}`)
   }
 
   // But we do not support prompt().
diff --git a/spec/chromium-spec.js b/spec/chromium-spec.js
index 6a5937bbce26..4c5b85521ac2 100644
--- a/spec/chromium-spec.js
+++ b/spec/chromium-spec.js
@@ -880,4 +880,28 @@ describe('chromium feature', function () {
       })
     })
   })
+
+  describe('window.alert(message, title)', function () {
+    it('throws an exception when the arguments cannot be converted to strings', function () {
+      assert.throws(function () {
+        window.alert({toString: null})
+      }, /Cannot convert object to primitive value/)
+
+      assert.throws(function () {
+        window.alert('message', {toString: 3})
+      }, /Cannot convert object to primitive value/)
+    })
+  })
+
+  describe('window.confirm(message, title)', function () {
+    it('throws an exception when the arguments cannot be converted to strings', function () {
+      assert.throws(function () {
+        window.confirm({toString: null}, 'title')
+      }, /Cannot convert object to primitive value/)
+
+      assert.throws(function () {
+        window.confirm('message', {toString: 3})
+      }, /Cannot convert object to primitive value/)
+    })
+  })
 })