diff --git a/lib/concurrent/agent.rb b/lib/concurrent/agent.rb
index 76a706b2e..9938d92cb 100644
--- a/lib/concurrent/agent.rb
+++ b/lib/concurrent/agent.rb
@@ -28,6 +28,7 @@ def initialize(initial, opts = {})
       @rescuers = []
       @validator = nil
       @timeout = opts[:timeout] || TIMEOUT
+      init_mutex
       set_deref_options(opts)
     end
 
diff --git a/lib/concurrent/contract.rb b/lib/concurrent/contract.rb
index 1b055098a..536818f24 100644
--- a/lib/concurrent/contract.rb
+++ b/lib/concurrent/contract.rb
@@ -7,6 +7,7 @@ class Contract
 
     def initialize(opts = {})
       @state = :pending
+      init_mutex
       set_deref_options(opts)
     end
 
diff --git a/lib/concurrent/dereferenceable.rb b/lib/concurrent/dereferenceable.rb
index 2de556fa3..5945ff39f 100644
--- a/lib/concurrent/dereferenceable.rb
+++ b/lib/concurrent/dereferenceable.rb
@@ -21,8 +21,8 @@ module Dereferenceable
     #   returning the value returned from the proc (default: `nil`)
     def set_deref_options(opts = {})
       mutex.synchronize do
-        @dup_on_deref = opts[:dup_on_deref] || opts[:dup] || false
-        @freeze_on_deref = opts[:freeze_on_deref] || opts[:freeze] || false
+        @dup_on_deref = opts[:dup_on_deref] || opts[:dup]
+        @freeze_on_deref = opts[:freeze_on_deref] || opts[:freeze]
         @copy_on_deref = opts[:copy_on_deref] || opts[:copy]
         @do_nothing_on_deref = ! (@dup_on_deref || @freeze_on_deref || @copy_on_deref)
       end
@@ -33,7 +33,7 @@ def set_deref_options(opts = {})
     def value
       return nil if @value.nil?
       return @value if @do_nothing_on_deref
-      return mutex.synchronize do
+      mutex.synchronize do
         value = @value
         value = @copy_on_deref.call(value) if @copy_on_deref
         value = value.dup if @dup_on_deref
@@ -45,9 +45,12 @@ def value
 
     protected
 
-    # @private
     def mutex # :nodoc:
-      @mutex ||= Mutex.new
+      @mutex
+    end
+
+    def init_mutex
+      @mutex = Mutex.new
     end
   end
 end
diff --git a/lib/concurrent/future.rb b/lib/concurrent/future.rb
index 76b97b1a1..fc78e132e 100644
--- a/lib/concurrent/future.rb
+++ b/lib/concurrent/future.rb
@@ -12,6 +12,7 @@ class Future
     include UsesGlobalThreadPool
 
     def initialize(*args, &block)
+      init_mutex
       unless block_given?
         @state = :fulfilled
       else
diff --git a/lib/concurrent/promise.rb b/lib/concurrent/promise.rb
index 0fbe1efc2..93624bc4f 100644
--- a/lib/concurrent/promise.rb
+++ b/lib/concurrent/promise.rb
@@ -41,6 +41,7 @@ def initialize(*args, &block)
       @children = []
       @rescuers = []
 
+      init_mutex
       realize(*args) if root?
     end
 
diff --git a/lib/concurrent/scheduled_task.rb b/lib/concurrent/scheduled_task.rb
index 6a1d2db80..30ed7fd06 100644
--- a/lib/concurrent/scheduled_task.rb
+++ b/lib/concurrent/scheduled_task.rb
@@ -29,6 +29,7 @@ def initialize(schedule_time, opts = {}, &block)
       @state = :pending
       @schedule_time.freeze
       @task = block
+      init_mutex
       set_deref_options(opts)
 
       @thread = Thread.new{ work }
diff --git a/lib/concurrent/timer_task.rb b/lib/concurrent/timer_task.rb
index abe1db0ef..b2a05c571 100644
--- a/lib/concurrent/timer_task.rb
+++ b/lib/concurrent/timer_task.rb
@@ -198,6 +198,7 @@ def initialize(opts = {}, &block)
       @run_now = opts[:now] || opts[:run_now] || false
 
       @task = block
+      init_mutex
       set_deref_options(opts)
     end