diff --git a/common/content/dactyl.js b/common/content/dactyl.js
index 018f32d8..8506b7d9 100644
--- a/common/content/dactyl.js
+++ b/common/content/dactyl.js
@@ -1319,6 +1319,8 @@ const Dactyl = Module("dactyl", {
                 let dialog = args[0];
 
                 dactyl.assert(dialog in config.dialogs, "E475: Invalid argument: " + dialog);
+                dactyl.assert(!config.dialogs[dialog][2] || config.dialogs[dialog][2](),
+                              "Dialog " + dialog + " not available");
                 try {
                     config.dialogs[dialog][1]();
                 }
@@ -1916,7 +1918,8 @@ const Dactyl = Module("dactyl", {
     completion: function () {
         completion.dialog = function dialog(context) {
             context.title = ["Dialog"];
-            context.completions = [[k, v[0]] for ([k, v] in Iterator(config.dialogs))];
+            context.filters.push(function ({ item }) !item[2] || item[2]());
+            context.completions = [[k, v[0], v[2]] for ([k, v] in Iterator(config.dialogs))];
         };
 
         completion.extension = function extension(context) {
diff --git a/pentadactyl/content/config.js b/pentadactyl/content/config.js
index 120b3a07..9631986e 100644
--- a/pentadactyl/content/config.js
+++ b/pentadactyl/content/config.js
@@ -46,7 +46,8 @@ const Config = Module("config", ConfigBase, {
         bookmarks: ["List your bookmarks",
             function () { window.openDialog("chrome://browser/content/bookmarks/bookmarksPanel.xul", "Bookmarks", "dialog,centerscreen,width=600,height=600"); }],
         checkupdates: ["Check for updates",
-            function () { window.checkForUpdates(); }],
+            function () { window.checkForUpdates(); },
+            function () "checkForUpdates" in window],
         cleardata: ["Clear private data",
             function () { Cc["@mozilla.org/browser/browserglue;1"].getService(Ci.nsIBrowserGlue).sanitize(window || null); }],
         cookies: ["List your cookies",
@@ -56,7 +57,8 @@ const Config = Module("config", ConfigBase, {
         customizetoolbar: ["Customize the Toolbar",
             function () { window.BrowserCustomizeToolbar(); }],
         dominspector: ["DOM Inspector",
-            function () { try { window.inspectDOMDocument(content.document); } catch (e) { dactyl.echoerr("DOM Inspector extension not installed"); } }],
+            function () { window.inspectDOMDocument(content.document); },
+            function () "inspectDOMDocument" in window],
         downloads: ["Manage Downloads",
             function () { window.toOpenWindowByType("Download:Manager", "chrome://mozapps/content/downloads/downloads.xul", "chrome,dialog=no,resizable"); }],
         history: ["List your history",
@@ -90,7 +92,8 @@ const Config = Module("config", ConfigBase, {
         selectionsource: ["View selection source",
             function () { buffer.viewSelectionSource(); }],
         venkman: ["The JavaScript debugger",
-            function () { dactyl.assert("start_venkman" in window, "Venkman is not installed"); start_venkman() }]
+            function () { dactyl.assert("start_venkman" in window, "Venkman is not installed"); start_venkman() },
+            function () "start_venkman" in window]
     },
 
     features: [