diff --git a/lib/concurrent-ruby/concurrent/set.rb b/lib/concurrent-ruby/concurrent/set.rb
index 602d49408..3bf0c895c 100644
--- a/lib/concurrent-ruby/concurrent/set.rb
+++ b/lib/concurrent-ruby/concurrent/set.rb
@@ -19,13 +19,19 @@ module Concurrent
   #
   #   @see http://ruby-doc.org/stdlib-2.4.0/libdoc/set/rdoc/Set.html Ruby standard library `Set`
 
-
   # @!macro internal_implementation_note
   SetImplementation = case
                       when Concurrent.on_cruby?
-                        # Because MRI never runs code in parallel, the existing
-                        # non-thread-safe structures should usually work fine.
-                        ::Set
+                        # The CRuby implementation of Set is written in Ruby itself and is
+                        # not thread safe for certain methods.
+                        require 'monitor'
+                        require 'concurrent/thread_safe/util/data_structures'
+
+                        class CRubySet < ::Set
+                        end
+
+                        ThreadSafe::Util.make_synchronized_on_cruby CRubySet
+                        CRubySet
 
                       when Concurrent.on_jruby?
                         require 'jruby/synchronized'
@@ -33,6 +39,7 @@ module Concurrent
                         class JRubySet < ::Set
                           include JRuby::Synchronized
                         end
+
                         JRubySet
 
                       when Concurrent.on_rbx?
@@ -41,7 +48,8 @@ class JRubySet < ::Set
 
                         class RbxSet < ::Set
                         end
-                        ThreadSafe::Util.make_synchronized_on_rbx Concurrent::RbxSet
+
+                        ThreadSafe::Util.make_synchronized_on_rbx RbxSet
                         RbxSet
 
                       when Concurrent.on_truffleruby?
@@ -50,7 +58,7 @@ class RbxSet < ::Set
                         class TruffleRubySet < ::Set
                         end
 
-                        ThreadSafe::Util.make_synchronized_on_truffleruby Concurrent::TruffleRubySet
+                        ThreadSafe::Util.make_synchronized_on_truffleruby TruffleRubySet
                         TruffleRubySet
 
                       else
diff --git a/lib/concurrent-ruby/concurrent/thread_safe/util/data_structures.rb b/lib/concurrent-ruby/concurrent/thread_safe/util/data_structures.rb
index ff1e8ed97..24d039b2f 100644
--- a/lib/concurrent-ruby/concurrent/thread_safe/util/data_structures.rb
+++ b/lib/concurrent-ruby/concurrent/thread_safe/util/data_structures.rb
@@ -12,12 +12,37 @@ def self.synchronized(object, &block)
 module Concurrent
   module ThreadSafe
     module Util
+      def self.make_synchronized_on_cruby(klass)
+        klass.class_eval do
+          def initialize(*args, &block)
+            @_monitor = Monitor.new
+            super
+          end
+
+          def initialize_copy(other)
+            # make sure a copy is not sharing a monitor with the original object!
+            @_monitor = Monitor.new
+            super
+          end
+        end
+
+        klass.superclass.instance_methods(false).each do |method|
+          klass.class_eval <<-RUBY, __FILE__, __LINE__ + 1
+            def #{method}(*args)
+              monitor = @_monitor
+              monitor or raise("BUG: Internal monitor was not properly initialized. Please report this to the concurrent-ruby developers.")
+              monitor.synchronize { super }
+            end
+          RUBY
+        end
+      end
+
       def self.make_synchronized_on_rbx(klass)
         klass.class_eval do
           private
 
           def _mon_initialize
-            @_monitor = Monitor.new unless @_monitor # avoid double initialisation
+            @_monitor ||= Monitor.new # avoid double initialisation
           end
 
           def self.new(*args)
diff --git a/lib/concurrent-ruby/concurrent/version.rb b/lib/concurrent-ruby/concurrent/version.rb
index 8d9dde235..af9e5a77f 100644
--- a/lib/concurrent-ruby/concurrent/version.rb
+++ b/lib/concurrent-ruby/concurrent/version.rb
@@ -1,3 +1,3 @@
 module Concurrent
-  VERSION      = '1.1.8'
+  VERSION = '1.1.8'
 end
diff --git a/spec/concurrent/set_spec.rb b/spec/concurrent/set_spec.rb
index 7c69b4bec..558801864 100644
--- a/spec/concurrent/set_spec.rb
+++ b/spec/concurrent/set_spec.rb
@@ -42,7 +42,7 @@ module Concurrent
     end
 
     context 'concurrency' do
-      it do
+      it '#add and #delete' do
         (1..Concurrent::ThreadSafe::Test::THREADS).map do |i|
           in_thread do
             1000.times do
@@ -55,6 +55,63 @@ module Concurrent
         end.map(&:join)
         expect(set).to be_empty
       end
+
+      it 'force context switch' do
+        barrier = Concurrent::CyclicBarrier.new(2)
+
+        # methods like include? or delete? are implemented for CRuby in Ruby itself
+        # @see https://github.com/ruby/ruby/blob/master/lib/set.rb
+        set.clear
+
+        # add a single element
+        set.add(1)
+
+        # This thread should start and `Set#reject!` in CRuby should cache a value of `0` for size
+        thread_reject = in_thread do
+          # we expect this to return nil since nothing should have changed.
+          expect(set.reject! do |v|
+            barrier.wait
+            v == 1 # only delete the 1 value
+          end).to eq set
+        end
+
+        thread_add = in_thread do
+          barrier.wait
+          expect(set.add?(1)).to eq set
+        end
+
+        join_with [thread_reject, thread_add]
+      end
+
+      it '#each' do
+        threads = []
+        ("a".."z").inject(set, &:<<) # setup a non-empty set
+
+        threads << in_thread do
+          2000.times do
+            size = nil
+            set.each do |member|
+              if size.nil?
+                size = set.length
+              else
+                expect(set.length).to eq(size)
+              end
+            end
+          end
+        end
+
+        threads += (1..19).map do |i|
+          in_thread do
+            v = i * 1000
+            10.times do
+              200.times { |j| set << (v+j) }
+              200.times { |j| set.delete(v+j) }
+            end
+          end
+        end
+
+        threads.map(&:join)
+      end
     end
   end
 end