diff --git a/lib/good_job.rb b/lib/good_job.rb
index 367f03cb4..1995888c3 100644
--- a/lib/good_job.rb
+++ b/lib/good_job.rb
@@ -131,7 +131,7 @@ def self.shutdown?
   # When forking processes you should shut down these background threads before forking, and restart them after forking.
   # For example, you should use +shutdown+ and +restart+ when using async execution mode with Puma.
   # See the {file:README.md#executing-jobs-async--in-process} for more explanation and examples.
-  # @param timeout [Numeric, nil] Seconds to wait for active threads to finish.
+  # @param timeout [Numeric] Seconds to wait for active threads to finish.
   # @return [void]
   def self.restart(timeout: -1)
     _shutdown_all(Capsule.instances, :restart, timeout: timeout)
diff --git a/lib/good_job/capsule.rb b/lib/good_job/capsule.rb
index a74d17673..c504c96e2 100644
--- a/lib/good_job/capsule.rb
+++ b/lib/good_job/capsule.rb
@@ -55,9 +55,11 @@ def shutdown(timeout: :default)
     end
 
     # Shutdown and then start the capsule again.
-    # @param timeout [nil, Numeric, Symbol] Seconds to wait for active threads.
+    # @param timeout [Numeric, Symbol] Seconds to wait for active threads.
     # @return [void]
     def restart(timeout: :default)
+      raise ArgumentError, "Capsule#restart cannot be called with a timeout of nil" if timeout.nil?
+
       shutdown(timeout: timeout)
       start
     end
diff --git a/lib/good_job/scheduler.rb b/lib/good_job/scheduler.rb
index cba1eb023..500f2d150 100644
--- a/lib/good_job/scheduler.rb
+++ b/lib/good_job/scheduler.rb
@@ -132,11 +132,13 @@ def shutdown(timeout: -1)
 
     # Restart the Scheduler.
     # When shutdown, start; or shutdown and start.
-    # @param timeout [nil, Numeric] Seconds to wait for actively executing jobs to finish; shares same values as {#shutdown}.
+    # @param timeout [Numeric] Seconds to wait for actively executing jobs to finish; shares same values as {#shutdown}.
     # @return [void]
     def restart(timeout: -1)
+      raise ArgumentError, "Scheduler#restart cannot be called with a timeout of nil" if timeout.nil?
+
       instrument("scheduler_restart_pools") do
-        shutdown(timeout: timeout) if running?
+        shutdown(timeout: timeout)
         create_executor
         warm_cache
       end
diff --git a/spec/lib/good_job/scheduler_spec.rb b/spec/lib/good_job/scheduler_spec.rb
index 6bb0d3fac..8bdec7b7c 100644
--- a/spec/lib/good_job/scheduler_spec.rb
+++ b/spec/lib/good_job/scheduler_spec.rb
@@ -114,6 +114,17 @@
       expect { scheduler.restart }
         .to change(scheduler, :running?).from(false).to(true)
     end
+
+    it 'can be called multiple times' do
+      scheduler = described_class.new(performer)
+      scheduler.shutdown
+      expect do
+        scheduler.restart
+        scheduler.restart
+        scheduler.restart
+      end.not_to raise_error
+      scheduler.shutdown
+    end
   end
 
   describe '#create_thread' do
@@ -144,13 +155,11 @@
     it 'uses state[:count] to create multiple threads' do
       job_performer = instance_double(GoodJob::JobPerformer, next: nil, next?: true, name: '', next_at: [], cleanup: nil)
       scheduler = described_class.new(job_performer, max_threads: 1)
+      allow(scheduler).to receive(:create_task)
 
       result = scheduler.create_thread({ count: 10 })
-      if result
-        expect(job_performer).to have_received(:next).exactly(10).times
-      else
-        expect(job_performer).to have_received(:next).at_most(9).times
-      end
+      expect(result).to be true
+      expect(scheduler).to have_received(:create_task).exactly(10).times
     end
   end
 
diff --git a/spec/lib/good_job_spec.rb b/spec/lib/good_job_spec.rb
index ab18d81d8..82623f856 100644
--- a/spec/lib/good_job_spec.rb
+++ b/spec/lib/good_job_spec.rb
@@ -13,7 +13,7 @@
   end
 
   describe '.shutdown?' do
-    it 'shuts down all scheduler and notifier instances' do
+    it 'returns whether any capsules are running' do
       expect do
         capsule = GoodJob::Capsule.new(configuration: configuration)
         capsule.start