diff --git a/lib/rack/cors.rb b/lib/rack/cors.rb
index 95eda57..80f5ac1 100644
--- a/lib/rack/cors.rb
+++ b/lib/rack/cors.rb
@@ -6,10 +6,12 @@ def initialize(app, opts={}, &block)
       @app = app
       @logger = opts[:logger]
 
-      if block.arity == 1
-        block.call(self)
-      else
-        instance_eval(&block)
+      if block_given?
+        if block.arity == 1
+          block.call(self)
+        else
+          instance_eval(&block)
+        end
       end
     end