just check to see that the first item in the list is an array

actionpack/lib/action_view/helpers/form_options_helper.rb

@@ -579,7 +579,7 @@ class InstanceTag #:nodoc:
       def to_select_tag(choices, options, html_options)
         selected_value = options.has_key?(:selected) ? options[:selected] : value(object)
 
-        if !choices.empty? && choices.try(:first).try(:second).respond_to?(:each)
+        if !choices.empty? && Array === choices.first
           option_tags = grouped_options_for_select(choices, :selected => selected_value, :disabled => options[:disabled])
         else
           option_tags = options_for_select(choices, :selected => selected_value, :disabled => options[:disabled])

actionpack/test/template/form_options_helper_test.rb

@@ -587,6 +587,15 @@ def test_select_with_prompt_and_blank
     )
   end
 
+  def test_empty
+    @post = Post.new
+    @post.category = ""
+    assert_dom_equal(
+      "<select id=\"post_category\" name=\"post[category]\"><option value=\"\">Please select</option>\n<option value=\"\"></option>\n</select>",
+      select("post", "category", [], :prompt => true, :include_blank => true)
+    )
+  end
+
   def test_select_with_selected_value
     @post = Post.new
     @post.category = "<mus>"