diff --git a/lib/rack/utf8_sanitizer.rb b/lib/rack/utf8_sanitizer.rb
index e36185d..bcf22e4 100644
--- a/lib/rack/utf8_sanitizer.rb
+++ b/lib/rack/utf8_sanitizer.rb
@@ -8,6 +8,7 @@ module Rack
   class UTF8Sanitizer
     StringIO = ::StringIO
     NULL_BYTE_REGEX = /\x00/.freeze
+    NULL_BYTE_STRING_REGEX = Regexp.new('\\\u0000').freeze
 
     class NullByteInString < StandardError; end
 
@@ -40,7 +41,7 @@ def call(env)
                   invalid: :replace,
                   undef:   :replace)
         if sanitize_null_bytes
-          input = input.gsub(NULL_BYTE_REGEX, "")
+          input = input.gsub(NULL_BYTE_REGEX, "").gsub(NULL_BYTE_STRING_REGEX, '')
         end
         input
       end,
@@ -48,7 +49,7 @@ def call(env)
         input.
           force_encoding(Encoding::ASCII_8BIT).
           encode!(Encoding::UTF_8)
-        if sanitize_null_bytes && NULL_BYTE_REGEX.match?(input)
+        if sanitize_null_bytes && (NULL_BYTE_REGEX.match?(input) || NULL_BYTE_STRING_REGEX.match?(input))
           raise NullByteInString
         end
         input
@@ -262,7 +263,8 @@ def sanitize_string(input)
       if input.is_a? String
         input = input.dup.force_encoding(Encoding::UTF_8)
 
-        if input.valid_encoding? && !(@sanitize_null_bytes && input =~ NULL_BYTE_REGEX)
+        if input.valid_encoding? &&
+          !(@sanitize_null_bytes && (NULL_BYTE_REGEX.match?(input) || NULL_BYTE_STRING_REGEX.match?(input)))
           input
         else
           @strategy.call(input, sanitize_null_bytes: @sanitize_null_bytes)
diff --git a/test/test_utf8_sanitizer.rb b/test/test_utf8_sanitizer.rb
index e3f4f87..0490fdd 100644
--- a/test/test_utf8_sanitizer.rb
+++ b/test/test_utf8_sanitizer.rb
@@ -395,6 +395,18 @@ def read
       end
     end
 
+    it "optionally sanitizes null bytes plain string with the replace strategy" do
+      @app = Rack::UTF8Sanitizer.new(-> env { env }, sanitize_null_bytes: true)
+      input = "foo=bla\xED&quux=bar" + '\u0000'
+      @rack_input = StringIO.new input
+
+      sanitize_form_data do |sanitized_input|
+        sanitized_input.encoding.should == Encoding::UTF_8
+        sanitized_input.should.be.valid_encoding
+        sanitized_input.should == "foo=bla%EF%BF%BD&quux=bar"
+      end
+    end
+
     it "optionally sanitizes encoded null bytes with the replace strategy" do
       @app = Rack::UTF8Sanitizer.new(-> env { env }, sanitize_null_bytes: true)
       input = "foo=bla%ED&quux=bar%00"