Don't look for attached nodes at invalid indices

spec/integration/session_spec.rb

@@ -60,6 +60,7 @@ describe Capybara::Session do
             <strong>Hello</strong>
             <span>UTF8文字列</span>
             <input type="button" value="ボタン" />
+            <a href="about:blank">Link</a>
           </body></html>
         HTML
         [200,
@@ -84,6 +85,13 @@ describe Capybara::Session do
     it "can click utf8 string" do
       subject.click_button('ボタン')
     end
+
+    it "raises an ElementNotFound error when the selector scope is no longer valid" do
+      subject.within('//body') do
+        subject.click_link 'Link'
+        lambda { subject.find('//strong') }.should raise_error(Capybara::ElementNotFound)
+      end
+    end
   end
 
   context "response headers with status code" do

src/capybara.js

@@ -51,7 +51,8 @@ Capybara = {
   },
 
   isAttached: function(index) {
-    return document.evaluate("ancestor-or-self::html", this.nodes[index], null, XPathResult.FIRST_ORDERED_NODE_TYPE, null).singleNodeValue != null;
+    return this.nodes[index] &&
+      document.evaluate("ancestor-or-self::html", this.nodes[index], null, XPathResult.FIRST_ORDERED_NODE_TYPE, null).singleNodeValue != null;
   },
 
   text: function (index) {