diff --git a/brozzler/behaviors.yaml b/brozzler/behaviors.yaml
index daf40f5..264a785 100644
--- a/brozzler/behaviors.yaml
+++ b/brozzler/behaviors.yaml
@@ -26,7 +26,7 @@
   behavior_js_template: umbraBehavior18q4.js.j2
   default_parameters:
     interval: 500
-    iframes: false
+    skip_iframes: true
     actions:
       - selector: button.coreSpriteDismissLarge
       - selector: 'a>.eLAPa>.KL4Bh'
diff --git a/brozzler/browser.py b/brozzler/browser.py
index 000ed8c..0548b1e 100644
--- a/brozzler/browser.py
+++ b/brozzler/browser.py
@@ -256,6 +256,8 @@ class WebsockReceiverThread(threading.Thread):
                 self.logger.debug(
                         'console.%s %s', message['params']['message']['level'],
                         message['params']['message']['text'])
+            elif message['method'] == 'Runtime.exceptionThrown':
+                self.logger.debug('uncaught exception: %s', message)
             elif message['method'] == 'Page.javascriptDialogOpening':
                 self._javascript_dialog_opening(message)
             elif (message['method'] == 'Network.loadingFailed'
diff --git a/brozzler/js-templates/umbraBehavior18q4.js.j2 b/brozzler/js-templates/umbraBehavior18q4.js.j2
index 100b5b7..7c24a13 100644
--- a/brozzler/js-templates/umbraBehavior18q4.js.j2
+++ b/brozzler/js-templates/umbraBehavior18q4.js.j2
@@ -26,7 +26,11 @@ class UmbraBehavior {
         this.idleSince = null;
         this.intervalId = null;
         this.intervalTimeMs = {{interval or 300}};
-        this.doIframes = {{iframes or true}};
+        {% if skip_iframes %}
+        this.skipIframes = true;
+        {% else %}
+        this.skipIframes = false;
+        {% endif %}
         this.index = 0;
     }
 
@@ -44,7 +48,7 @@ class UmbraBehavior {
 
         var documents = [];
         documents[0] = document;
-        if (this.doIframes) {
+        if (!(this.skipIframes)) {
             var iframes = document.querySelectorAll("iframe");
             var iframesLength = iframes.length;
             for (var i = 0; i < iframesLength; i++) {
diff --git a/brozzler/worker.py b/brozzler/worker.py
index 3cfa9fc..fba83aa 100644
--- a/brozzler/worker.py
+++ b/brozzler/worker.py
@@ -278,11 +278,13 @@ class BrozzlerWorker:
         def _on_service_worker_version_updated(chrome_msg):
             # https://github.com/internetarchive/brozzler/issues/140
             self.logger.trace('%r', chrome_msg)
-            url = chrome_msg.get('params', {}).get('versions', [{}])[0].get('scriptURL')
-            if url not in sw_fetched:
-                self.logger.info('fetching service worker script %s', url)
-                self._fetch_url(site, url)
-                sw_fetched.add(url)
+            if chrome_msg.get('params', {}).get('versions'):
+                url = chrome_msg.get('params', {}).get('versions')[0]\
+                        .get('scriptURL')
+                if url and url not in sw_fetched:
+                    self.logger.info('fetching service worker script %s', url)
+                    self._fetch_url(site, url)
+                    sw_fetched.add(url)
 
         if not browser.is_running():
             browser.start(
diff --git a/setup.py b/setup.py
index bc4b868..1fb7ff8 100755
--- a/setup.py
+++ b/setup.py
@@ -32,7 +32,7 @@ def find_package_data(package):
 
 setuptools.setup(
         name='brozzler',
-        version='1.5.dev315',
+        version='1.5.dev317',
         description='Distributed web crawling with browsers',
         url='https://github.com/internetarchive/brozzler',
         author='Noah Levitt',