diff --git a/sentry-ruby/lib/sentry-ruby.rb b/sentry-ruby/lib/sentry-ruby.rb
index 6dcf99671..4ebf5b299 100644
--- a/sentry-ruby/lib/sentry-ruby.rb
+++ b/sentry-ruby/lib/sentry-ruby.rb
@@ -50,6 +50,8 @@ module Sentry
 
   THREAD_LOCAL = :sentry_hub
 
+  MUTEX = Mutex.new
+
   class << self
     # @!visibility private
     def exception_locals_tp
@@ -275,8 +277,10 @@ def close
 
       @background_worker.shutdown
 
-      @main_hub = nil
-      Thread.current.thread_variable_set(THREAD_LOCAL, nil)
+      MUTEX.synchronize do
+        @main_hub = nil
+        Thread.current.thread_variable_set(THREAD_LOCAL, nil)
+      end
     end
 
     # Returns true if the SDK is initialized.
@@ -303,7 +307,7 @@ def csp_report_uri
     #
     # @return [Hub]
     def get_main_hub
-      @main_hub
+      MUTEX.synchronize { @main_hub }
     end
 
     # Takes an instance of Sentry::Breadcrumb and stores it to the current active scope.
diff --git a/sentry-sidekiq/Gemfile b/sentry-sidekiq/Gemfile
index c158e85dc..c860cb206 100644
--- a/sentry-sidekiq/Gemfile
+++ b/sentry-sidekiq/Gemfile
@@ -12,9 +12,6 @@ gem "sentry-rails", path: "../sentry-rails"
 # loofah changed the required ruby version in a patch so we need to explicitly pin it
 gem "loofah", "2.20.0" if RUBY_VERSION.to_f < 2.5
 
-# For https://github.com/ruby/psych/issues/655
-gem "psych", "5.1.0"
-
 sidekiq_version = ENV["SIDEKIQ_VERSION"]
 sidekiq_version = "7.0" if sidekiq_version.nil?
 sidekiq_version = Gem::Version.new(sidekiq_version)