properly handle lists of lists. Thanks @adrianpike for reporting!

actionpack/lib/action_view/helpers/form_options_helper.rb

@@ -579,7 +579,12 @@ module ActionView
       def to_select_tag(choices, options, html_options)
         selected_value = options.has_key?(:selected) ? options[:selected] : value(object)
 
-        if !choices.empty? && Array === choices.first
+        # Grouped choices look like this:
+        #
+        #   [nil, []]
+        #   { nil => [] }
+        #
+        if !choices.empty? && choices.first.last.respond_to?(:each)
           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

@@ -596,6 +596,15 @@ class FormOptionsHelperTest < ActionView::TestCase
     )
   end
 
+  def test_list_of_lists
+    @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<option value=\"number\">Number</option>\n<option value=\"text\">Text</option>\n<option value=\"boolean\">Yes/No</option></select>",
+      select("post", "category", [["Number", "number"], ["Text", "text"], ["Yes/No", "boolean"]], :prompt => true, :include_blank => true)
+    )
+  end
+
   def test_select_with_selected_value
     @post = Post.new
     @post.category = "<mus>"