Properly handle nil image path

features/responsive-image-block.feature

@@ -38,3 +38,13 @@ Feature: Jekyll responsive_image_block tag
       """
     When I run Jekyll
     Then I should see "<img alt=\"Lorem ipsum\" src=\"/assets/test.png\"" in "_site/index.html"
+
+  Scenario: Handling a nil path
+    Given I have a responsive_image configuration with "template" set to "_includes/responsive-image.html"
+    And I have a file "index.html" with:
+      """
+      {% responsive_image_block %}
+          path: {{ path }}
+      {% endresponsive_image_block %}
+      """
+    Then Jekyll should throw a "SyntaxError"

features/step_definitions/jekyll_steps.rb

@@ -1,7 +1,13 @@
+include Test::Unit::Assertions
+
 When /^I run Jekyll$/ do
   run_jekyll
 end
 
+Then /^Jekyll should throw a "(.+)"$/ do |error_class|
+  assert_raise(Object.const_get(error_class)) { run_jekyll }
+end
+
 Given /^I have a responsive_image configuration with:$/ do |config|
   write_file('_config.yml', "responsive_image:\n#{config}")
 end

lib/jekyll/responsive_image/image_processor.rb

@@ -4,7 +4,7 @@ module Jekyll
       include ResponsiveImage::Utils
 
       def process(image_path, config)
-        raise SyntaxError.new('Invalid image path specified') unless File.exists?(image_path)
+        raise SyntaxError.new('Invalid image path specified') unless File.exists?(image_path.to_s)
 
         resize_handler = ResizeHandler.new
         img = Magick::Image::read(image_path).first