diff --git a/lib/mega_mutex.rb b/lib/mega_mutex.rb
index 833536f..bbaaa52 100644
--- a/lib/mega_mutex.rb
+++ b/lib/mega_mutex.rb
@@ -104,4 +104,3 @@ def configuration
     end
   end
 end
-
diff --git a/lib/mega_mutex/distributed_mutex.rb b/lib/mega_mutex/distributed_mutex.rb
index 2300baa..6e194be 100644
--- a/lib/mega_mutex/distributed_mutex.rb
+++ b/lib/mega_mutex/distributed_mutex.rb
@@ -74,9 +74,9 @@ def locked_by_me?
     end
 
     def set_current_lock(new_lock)
-      cache.set(@key, my_lock_id)
-      # expire redis key after 1 hour
-      cache.expire(@key, 3600)
+      # nx: only set key if it doesn't exist
+      # px: expire redis key after 1 hour (in milliseconds)
+      cache.set(@key, my_lock_id, nx: true, px: 3_600_000)
     end
 
     def my_lock_id
diff --git a/mega_mutex.gemspec b/mega_mutex.gemspec
index 92a6747..29895e5 100644
--- a/mega_mutex.gemspec
+++ b/mega_mutex.gemspec
@@ -42,6 +42,6 @@ Gem::Specification.new do |s|
   s.add_runtime_dependency(%q<redis>, ["~> 4"])
   s.add_runtime_dependency(%q<logging>, [">= 1.1.4"])
   s.add_development_dependency("rspec", ["= 1.3.0"])
-  s.add_development_dependency("rake", [">= 1.0"])
+  s.add_development_dependency("rake", ["< 11.0"])
 
 end
diff --git a/spec/lib/mega_mutex_spec.rb b/spec/lib/mega_mutex_spec.rb
index 182ff55..86d0fc2 100644
--- a/spec/lib/mega_mutex_spec.rb
+++ b/spec/lib/mega_mutex_spec.rb
@@ -1,4 +1,4 @@
-require File.dirname(__FILE__) + '/../spec_helper'
+require_relative '../spec_helper'
 
 module MegaMutex
   describe MegaMutex do
@@ -39,7 +39,7 @@ def logger
 
       describe "with the same lock key" do
         before(:each) do
-          Redis.new({:host => 'redis.dev', :port => 6379}).delete(mutex_id)
+          Redis.new({:host => 'redis.dev', :port => 6379}).del(mutex_id)
         end
 
         def mutex_id
@@ -106,7 +106,6 @@ def mutex_id
     end
 
     describe "with a timeout" do
-
       it "should raise an error if the code blocks for longer than the timeout" do
         @exception = nil
         @first_thread_has_started = false
@@ -126,7 +125,8 @@ def mutex_id
           end
         end
         wait_for_threads_to_finish
-        assert @exception.is_a?(MegaMutex::TimeoutError), "Expected TimeoutError to be raised, but wasn't"
+
+        @exception.should be_kind_of(MegaMutex::TimeoutError), "Expected TimeoutError to be raised, but wasn't"
       end
     end
   end
diff --git a/spec/spec_helper.rb b/spec/spec_helper.rb
index 7044eff..a225ca6 100644
--- a/spec/spec_helper.rb
+++ b/spec/spec_helper.rb
@@ -1,5 +1,4 @@
-require File.expand_path(File.dirname(__FILE__) + '/../lib/mega_mutex')
-require 'test/unit/assertions'
+require_relative '../lib/mega_mutex'
 
 # Logging::Logger[:root].add_appenders(Logging::Appenders.stdout)
 
@@ -28,5 +27,4 @@ def wait_for_threads_to_finish
 Spec::Runner.configure do |config|
   config.extend ThreadHelper
   config.include ThreadExampleHelper
-  config.include Test::Unit::Assertions
 end