diff --git a/lib/capybara/selector/css.rb b/lib/capybara/selector/css.rb
index 33ad1b53..58a57476 100644
--- a/lib/capybara/selector/css.rb
+++ b/lib/capybara/selector/css.rb
@@ -31,22 +31,22 @@ module Capybara
         def split(css)
           selectors = []
           StringIO.open(css.to_s) do |str|
-            selector = ''
+            selector = +''
             while (char = str.getc)
               case char
               when '['
-                selector += parse_square(str)
+                selector << parse_square(str)
               when '('
-                selector += parse_paren(str)
+                selector << parse_paren(str)
               when '"', "'"
-                selector += parse_string(char, str)
+                selector << parse_string(char, str)
               when '\\'
-                selector += char + str.getc
+                selector << char + str.getc
               when ','
                 selectors << selector.strip
-                selector = ''
+                selector.clear
               else
-                selector += char
+                selector << char
               end
             end
             selectors << selector.strip
diff --git a/lib/capybara/selector/filter_set.rb b/lib/capybara/selector/filter_set.rb
index 9f3f744a..16365385 100644
--- a/lib/capybara/selector/filter_set.rb
+++ b/lib/capybara/selector/filter_set.rb
@@ -40,9 +40,9 @@ module Capybara
       def description(node_filters: true, expression_filters: true, **options)
         opts = options_with_defaults(options)
         description = +''
-        description += undeclared_descriptions.map { |desc| desc.call(opts).to_s }.join
-        description += expression_filter_descriptions.map { |desc| desc.call(opts).to_s }.join if expression_filters
-        description += node_filter_descriptions.map { |desc| desc.call(opts).to_s }.join if node_filters
+        description << undeclared_descriptions.map { |desc| desc.call(opts).to_s }.join
+        description << expression_filter_descriptions.map { |desc| desc.call(opts).to_s }.join if expression_filters
+        description << node_filter_descriptions.map { |desc| desc.call(opts).to_s }.join if node_filters
         description
       end