diff --git a/.travis.yml b/.travis.yml
index 90ce570..a943bf0 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -2,3 +2,6 @@ language: node_js
 node_js:
     - "0.8"
     - "0.10"
+    - "4"
+    - "5"
+    - "6"
diff --git a/index.js b/index.js
index aef07e2..97d8935 100644
--- a/index.js
+++ b/index.js
@@ -214,6 +214,7 @@ function resolve(id, opts, cb) {
     // opts.packageFilter
 
     opts = opts || {};
+    opts.filename = opts.filename || '';
 
     var base = path.dirname(opts.filename);
 
@@ -278,6 +279,8 @@ resolve.sync = function (id, opts) {
     // opts.packageFilter
 
     opts = opts || {};
+    opts.filename = opts.filename || '';
+
     var base = path.dirname(opts.filename);
 
     if (opts.basedir) {