diff --git a/activesupport/lib/active_support/inflector/methods.rb b/activesupport/lib/active_support/inflector/methods.rb
index c68a2baee730a..1dc2e0487f2dc 100644
--- a/activesupport/lib/active_support/inflector/methods.rb
+++ b/activesupport/lib/active_support/inflector/methods.rb
@@ -94,7 +94,10 @@ def humanize(lower_case_and_underscored_word)
       result = lower_case_and_underscored_word.to_s.dup
       inflections.humans.each { |(rule, replacement)| break if result.gsub!(rule, replacement) }
       result.gsub!(/_id$/, "")
-      result.gsub(/(_)?([a-z\d]*)/i) { "#{$1 && ' '}#{inflections.acronyms[$2] || $2.downcase}" }.gsub(/^\w/) { $&.upcase }
+      result.gsub!(/_/, ' ')
+      result.gsub(/([a-z\d]*)/i) { |match|
+        "#{inflections.acronyms[match] || match.downcase}"
+      }.gsub(/^\w/) { $&.upcase }
     end
 
     # Capitalizes all the words and replaces some characters in the string to create
diff --git a/activesupport/test/safe_buffer_test.rb b/activesupport/test/safe_buffer_test.rb
index 8f77999d25159..db9159b289518 100644
--- a/activesupport/test/safe_buffer_test.rb
+++ b/activesupport/test/safe_buffer_test.rb
@@ -12,6 +12,10 @@ def setup
     @buffer = ActiveSupport::SafeBuffer.new
   end
 
+  def test_titleize
+    assert_equal 'Foo', "foo".html_safe.titleize
+  end
+
   test "Should look like a string" do
     assert @buffer.is_a?(String)
     assert_equal "", @buffer