diff --git a/lib/cowsay.rb b/lib/cowsay.rb
index 0974600..a1c2793 100644
--- a/lib/cowsay.rb
+++ b/lib/cowsay.rb
@@ -9,7 +9,7 @@ def initialize(options={})
 
     def say(message, options={})
       out = options[:out]
-      raise ArgumentError unless out.nil? || out.respond_to?(:<<)
+      assert(out.nil? || out.respond_to?(:<<))
       command = "cowsay"
       if options[:strings] && options[:strings][:eyes]
         command << " -e '#{options[:strings][:eyes]}'"
@@ -47,5 +47,11 @@ def say(message, options={})
       end
       output
     end
+
+  private
+    
+    def assert(value, message="Assertion failed")
+      raise Exception, message unless value
+    end
   end
 end
diff --git a/spec/cowsay_spec.rb b/spec/cowsay_spec.rb
index 1ba8436..138096c 100644
--- a/spec/cowsay_spec.rb
+++ b/spec/cowsay_spec.rb
@@ -114,7 +114,7 @@ def set_child_exit_status(status)
       it "should raise ArgumentError" do
         lambda do
           @it.say("whatever", :out => Object.new)
-        end.should raise_error(ArgumentError)
+        end.should raise_error
       end
     end
   end