diff --git a/activesupport/lib/active_support/json/encoders/string.rb b/activesupport/lib/active_support/json/encoders/string.rb
index 28f1190662..20ca91f9aa 100644
--- a/activesupport/lib/active_support/json/encoders/string.rb
+++ b/activesupport/lib/active_support/json/encoders/string.rb
@@ -19,9 +19,11 @@ end
 
 class String
   def to_json(options = nil) #:nodoc:
-    '"' + gsub(/[\010\f\n\r\t"\\><&]/) { |s|
+    json = '"' + gsub(/[\010\f\n\r\t"\\><&]/) { |s|
       ActiveSupport::JSON::Encoding::ESCAPED_CHARS[s]
-    }.gsub(/([\xC0-\xDF][\x80-\xBF]|
+    }
+    json.force_encoding('ascii-8bit') if respond_to?(:force_encoding)
+    json.gsub(/([\xC0-\xDF][\x80-\xBF]|
              [\xE0-\xEF][\x80-\xBF]{2}|
              [\xF0-\xF7][\x80-\xBF]{3})+/nx) { |s|
       s.unpack("U*").pack("n*").unpack("H*")[0].gsub(/.{4}/, '\\\\u\&')