diff --git a/lib/runnable.js b/lib/runnable.js
index 64de4e9a20..bba47d9542 100644
--- a/lib/runnable.js
+++ b/lib/runnable.js
@@ -96,7 +96,7 @@ Runnable.prototype.timeout = function(ms) {
  * @return {Runnable|number} ms or Runnable instance.
  */
 Runnable.prototype.slow = function(ms) {
-  if (!arguments.length) {
+  if (typeof ms === 'undefined') {
     return this._slow;
   }
   if (typeof ms === 'string') {
diff --git a/test/runnable.js b/test/runnable.js
index af34d06474..a3d88adda2 100644
--- a/test/runnable.js
+++ b/test/runnable.js
@@ -49,12 +49,27 @@ describe('Runnable(title, fn)', function(){
     });
   });
 
-  describe('#slow(ms)', function(){
-    it('should set the slow threshold', function(){
-      var run = new Runnable;
-      run.slow(100)
+  describe('#slow(ms)', function() {
+    var run;
+
+    beforeEach(function() {
+      run = new Runnable();
+    });
+
+    it('should set the slow threshold', function() {
+      run.slow(100);
       run.slow().should.equal(100);
-    })
+    });
+
+    it('should not set the slow threshold if the parameter is not passed', function() {
+      run.slow();
+      run.slow().should.equal(75);
+    });
+
+    it('should not set the slow threshold if the parameter is undefined', function() {
+      run.slow(undefined);
+      run.slow().should.equal(75);
+    });
   })
 
   describe('.title', function(){