diff --git a/lib/concurrent/executor/timer_set.rb b/lib/concurrent/executor/timer_set.rb
index 27239c898..e93e703b5 100644
--- a/lib/concurrent/executor/timer_set.rb
+++ b/lib/concurrent/executor/timer_set.rb
@@ -22,10 +22,10 @@ class TimerSet
     # @option opts [object] :executor when provided will run all operations on
     #   this executor rather than the global thread pool (overrides :operation)
     def initialize(opts = {})
-      @queue = PriorityQueue.new(order: :min)
-      @task_executor = OptionsParser::get_executor_from(opts) || Concurrent.configuration.global_task_pool
+      @queue          = PriorityQueue.new(order: :min)
+      @task_executor  = OptionsParser::get_executor_from(opts) || Concurrent.configuration.global_task_pool
       @timer_executor = SingleThreadExecutor.new
-      @condition = Condition.new
+      @condition      = Condition.new
       init_executor
     end
 
@@ -64,7 +64,7 @@ def post(intended_time, *args, &task)
     # For a timer, #kill is like an orderly shutdown, except we need to manually
     # (and destructively) clear the queue first
     def kill
-      @queue.clear
+      mutex.synchronize { @queue.clear }
       shutdown
     end
 
@@ -124,14 +124,13 @@ def shutdown_execution
     # @!visibility private
     def process_tasks
       loop do
-        break if @queue.empty?
-
-        task = @queue.peek
+        task = mutex.synchronize { @queue.peek }
+        break unless task
         interval = task.time - Time.now.to_f
 
         if interval <= 0
           @task_executor.post(*task.args, &task.op)
-          @queue.pop
+          mutex.synchronize { @queue.pop }
         else
           mutex.synchronize do
             @condition.wait(mutex, [interval, 60].min)