diff --git a/lib/ferrum_pdf/html_preprocessor.rb b/lib/ferrum_pdf/html_preprocessor.rb
index 8f69f38..ce9ea84 100644
--- a/lib/ferrum_pdf/html_preprocessor.rb
+++ b/lib/ferrum_pdf/html_preprocessor.rb
@@ -9,6 +9,8 @@ module HTMLPreprocessor
     #   process("Some HTML", "https://example.org")
     #
     def self.process(html, base_url)
+      return html if base_url.blank?
+
       base_url += "/" unless base_url.end_with? "/"
       protocol = base_url.split("://").first
       html = translate_relative_paths(html, base_url) if base_url
diff --git a/test/html_preprocessing_test.rb b/test/html_preprocessing_test.rb
index 34f116e..06be7f0 100644
--- a/test/html_preprocessing_test.rb
+++ b/test/html_preprocessing_test.rb
@@ -5,6 +5,11 @@ class HtmlPreProcessingTest< ActiveSupport::TestCase
     assert_equal "<h1>Hello</h1>", FerrumPdf::HTMLPreprocessor.process("<h1>Hello</h1>", "https://example.org")
   end
 
+  test "does not raise error when url absent" do
+    assert_equal "<a href=\"/example.pdf\">PDF</a>", FerrumPdf::HTMLPreprocessor.process("<a href=\"/example.pdf\">PDF</a>", nil)
+    assert_equal "<a href=\"/example.pdf\">PDF</a>", FerrumPdf::HTMLPreprocessor.process("<a href=\"/example.pdf\">PDF</a>", "")
+  end
+
   test "replaces relative paths" do
     assert_equal "<a href=\"http://example.org/example.pdf\">PDF</a>", FerrumPdf::HTMLPreprocessor.process("<a href=\"/example.pdf\">PDF</a>", "http://example.org")
     assert_equal "<a href=\"https://example.com/example.pdf\">PDF</a>", FerrumPdf::HTMLPreprocessor.process("<a href=\"/example.pdf\">PDF</a>", "https://example.com")