diff --git a/lib/vows.js b/lib/vows.js
index b8390ae..e68d5ec 100644
--- a/lib/vows.js
+++ b/lib/vows.js
@@ -75,7 +75,7 @@ function addVow(/* description & callback */) {
         // Sometimes it might be nicer to pass the proof first,
         // and the description second, so we let the user choose
         // the order.
-        if (args[0] instanceof Function) {
+        if (typeof(args[0]) === "function") {
             vow.callback = args[0], vow.description = args[1];
         } else {
             vow.callback = args[1], vow.description = args[0];
@@ -302,7 +302,7 @@ vows.tell = function (topic, tests) {
                     // setup fires.
                     // If we encounter an object literal, we recurse, sending it
                     // our current context.
-                    if (vow.callback instanceof Function) {
+                    if (typeof(vow.callback) === 'function') {
                         setup.addVow(vow);
                     } else if (typeof(vow.callback) === 'object' && ! Array.isArray(vow.callback)) {
                         // If there's a setup stage, we have to wait for it to fire,