diff --git a/13_integrated_testing/README.md b/13_integrated_testing/README.md
index ed84d533..755576f9 100644
--- a/13_integrated_testing/README.md
+++ b/13_integrated_testing/README.md
@@ -1938,7 +1938,7 @@ diff -uNr 12_exceptions_part1_groundwork/tests/runner.rb 13_integrated_testing/t
 +        puts "#{INDENT}-------------------------------------------------------------------"
 +        print INDENT
 +        print '🦀 '
-+        print @output.join('').gsub("\n", "\n#{INDENT}")
++        print @output.join.gsub("\n", "\n#{INDENT}")
 +    end
 +
 +    def finish(error)
diff --git a/13_integrated_testing/tests/runner.rb b/13_integrated_testing/tests/runner.rb
index 5b012304..2a0aa393 100755
--- a/13_integrated_testing/tests/runner.rb
+++ b/13_integrated_testing/tests/runner.rb
@@ -32,7 +32,7 @@ class Test
         puts "#{INDENT}-------------------------------------------------------------------"
         print INDENT
         print '🦀 '
-        print @output.join('').gsub("\n", "\n#{INDENT}")
+        print @output.join.gsub("\n", "\n#{INDENT}")
     end
 
     def finish(error)
diff --git a/14_exceptions_part2_peripheral_IRQs/tests/runner.rb b/14_exceptions_part2_peripheral_IRQs/tests/runner.rb
index 5b012304..2a0aa393 100755
--- a/14_exceptions_part2_peripheral_IRQs/tests/runner.rb
+++ b/14_exceptions_part2_peripheral_IRQs/tests/runner.rb
@@ -32,7 +32,7 @@ class Test
         puts "#{INDENT}-------------------------------------------------------------------"
         print INDENT
         print '🦀 '
-        print @output.join('').gsub("\n", "\n#{INDENT}")
+        print @output.join.gsub("\n", "\n#{INDENT}")
     end
 
     def finish(error)
diff --git a/15_virtual_mem_part2_mmio_remap/tests/runner.rb b/15_virtual_mem_part2_mmio_remap/tests/runner.rb
index 5b012304..2a0aa393 100755
--- a/15_virtual_mem_part2_mmio_remap/tests/runner.rb
+++ b/15_virtual_mem_part2_mmio_remap/tests/runner.rb
@@ -32,7 +32,7 @@ class Test
         puts "#{INDENT}-------------------------------------------------------------------"
         print INDENT
         print '🦀 '
-        print @output.join('').gsub("\n", "\n#{INDENT}")
+        print @output.join.gsub("\n", "\n#{INDENT}")
     end
 
     def finish(error)
diff --git a/Gemfile b/Gemfile
index 13e8f5d9..b6fad4ea 100644
--- a/Gemfile
+++ b/Gemfile
@@ -10,5 +10,5 @@ group :uart do
 end
 
 group :development do
-    gem 'rubocop', '>= 1.3.0', require: false
+    gem 'rubocop', '>= 1.4.1', require: false
 end