diff --git a/index.js b/index.js
index eb6ba89e..b7c9bf33 100644
--- a/index.js
+++ b/index.js
@@ -1,7 +1,6 @@
-var core = require('./lib/core');
 var async = require('./lib/async');
-async.core = core;
-async.isCore = function isCore(x) { return core[x]; };
+async.core = require('./lib/core');
+async.isCore = require('./lib/is-core');
 async.sync = require('./lib/sync');
 
 exports = async;
diff --git a/lib/async.js b/lib/async.js
index 513c8282..4ea25732 100644
--- a/lib/async.js
+++ b/lib/async.js
@@ -1,9 +1,9 @@
-var core = require('./core');
 var fs = require('fs');
 var path = require('path');
 var caller = require('./caller.js');
 var nodeModulesPaths = require('./node-modules-paths.js');
 var normalizeOptions = require('./normalize-options.js');
+var isCore = require('./is-core');
 
 var defaultIsFile = function isFile(file, cb) {
     fs.stat(file, function (err, stat) {
@@ -84,7 +84,7 @@ module.exports = function resolve(x, options, callback) {
             } else loadAsFile(res, opts.package, onfile);
         } else loadNodeModules(x, basedir, function (err, n, pkg) {
             if (err) cb(err);
-            else if (core[x]) return cb(null, x);
+            else if (isCore(x)) return cb(null, x);
             else if (n) {
                 return maybeUnwrapSymlink(n, opts, function (err, realN) {
                     if (err) {
diff --git a/lib/is-core.js b/lib/is-core.js
new file mode 100644
index 00000000..48bc96c4
--- /dev/null
+++ b/lib/is-core.js
@@ -0,0 +1,5 @@
+var core = require('./core');
+
+module.exports = function isCore(x) {
+    return Object.prototype.hasOwnProperty.call(core, x);
+};
diff --git a/lib/sync.js b/lib/sync.js
index 02ab67be..b6a17e2e 100644
--- a/lib/sync.js
+++ b/lib/sync.js
@@ -1,4 +1,4 @@
-var core = require('./core');
+var isCore = require('./is-core');
 var fs = require('fs');
 var path = require('path');
 var caller = require('./caller.js');
@@ -62,14 +62,14 @@ module.exports = function (x, options) {
         if (x === '..' || x.slice(-1) === '/') res += '/';
         var m = loadAsFileSync(res) || loadAsDirectorySync(res);
         if (m) return maybeUnwrapSymlink(m, opts);
-    } else if (core[x]) {
+    } else if (isCore(x)) {
         return x;
     } else {
         var n = loadNodeModulesSync(x, absoluteStart);
         if (n) return maybeUnwrapSymlink(n, opts);
     }
 
-    if (core[x]) return x;
+    if (isCore(x)) return x;
 
     var err = new Error("Cannot find module '" + x + "' from '" + parent + "'");
     err.code = 'MODULE_NOT_FOUND';
diff --git a/test/core.js b/test/core.js
index 33d9f329..4c111e1d 100644
--- a/test/core.js
+++ b/test/core.js
@@ -10,6 +10,9 @@ test('core modules', function (t) {
 
         st.ok(!resolve.isCore('seq'));
         st.ok(!resolve.isCore('../'));
+
+        st.ok(!resolve.isCore('toString'));
+
         st.end();
     });