diff --git a/lib/core.js b/lib/core.js
index dcd2065..ea26d6d 100644
--- a/lib/core.js
+++ b/lib/core.js
@@ -10,7 +10,9 @@ var util = require('util');
 function promisifyAll(obj) {
   var aobj = {};
   Object.keys(obj).forEach(function (key) {
-    aobj[key + 'Async'] = util.promisify(obj[key]);
+    if ('function' === typeof obj[key]) {
+      aobj[key + 'Async'] = util.promisify(obj[key]);
+    }
   });
   return aobj;
 }