Better error message if response.body is blank or not parseable by Nokogiri

Author: ghiculescu

lib/rails/dom/testing/assertions/selector_assertions.rb

@@ -4,6 +4,8 @@
 module Rails
   module Dom
     module Testing
+      MissingNodeError = Class.new(StandardError)
+
       module Assertions
         # Adds the +assert_dom+ method for use in Rails functional
         # test cases, which can be used to make assertions on the response HTML of a controller
@@ -296,8 +298,10 @@ def nest_selection(selection)
           def nodeset(node)
             if node.is_a?(Nokogiri::XML::NodeSet)
               node
-            else
+            elsif node.present?
               Nokogiri::XML::NodeSet.new(node.document, [node])
+            else
+              raise MissingNodeError, 'No node/document found. Did you return a blank body in your response?'
             end
           end
         end

test/selector_assertions_test.rb

@@ -322,6 +322,16 @@ def test_assert_select_with_extra_argument
     assert_select "title", text: "Welcome", count: 1
   end
 
+  def test_assert_select_on_blank_response
+    render_html ""
+    assert_raises Rails::Dom::Testing::MissingNodeError do
+      assert_select "div", 0
+    end
+    assert_raises Rails::Dom::Testing::MissingNodeError do
+      assert_select "div", 1
+    end
+  end
+
   protected
     def render_html(html)
       fake_render(:html, html)