diff --git a/lib/split/helper.rb b/lib/split/helper.rb
index 794e1f8e..61adc464 100644
--- a/lib/split/helper.rb
+++ b/lib/split/helper.rb
@@ -166,7 +166,7 @@ def control_variable(control)
 
     def start_trial(trial)
       experiment = trial.experiment
-      if override_present?(experiment.name)
+      if override_present?(experiment.name) and experiment[override_alternative(experiment.name)]
         ret = override_alternative(experiment.name)
         ab_user[experiment.key] = ret if Split.configuration.store_override
       elsif experiment.has_winner?
diff --git a/spec/helper_spec.rb b/spec/helper_spec.rb
index baac05c8..2455aa43 100755
--- a/spec/helper_spec.rb
+++ b/spec/helper_spec.rb
@@ -101,6 +101,12 @@
       alternative.should eql('red')
     end
 
+    it "should not allow an arbitrary alternative" do
+      @params = {'link_color' => 'pink'}
+      alternative = ab_test('link_color', 'blue')
+      alternative.should eql('blue')
+    end
+
     it "should not store the split when a param forced alternative" do
       @params = {'link_color' => 'blue'}
       ab_user.should_not_receive(:[]=)