diff --git a/app/models/solid_queue/job/concurrency_controls.rb b/app/models/solid_queue/job/concurrency_controls.rb
index b59c464ad..33610ca1b 100644
--- a/app/models/solid_queue/job/concurrency_controls.rb
+++ b/app/models/solid_queue/job/concurrency_controls.rb
@@ -8,7 +8,7 @@ module ConcurrencyControls
       included do
         has_one :blocked_execution
 
-        delegate :concurrency_limit, :concurrency_duration, to: :job_class
+        delegate :concurrency_limit, :concurrency_duration, to: :job_class, allow_nil: true
 
         before_destroy :unblock_next_blocked_job, if: -> { concurrency_limited? && ready? }
       end
@@ -48,8 +48,10 @@ def acquire_concurrency_lock
           Semaphore.wait(self)
         end
 
+        # A job whose class can't be loaded where it runs still holds the lock its
+        # key took when it was dispatched, so release by key alone.
         def release_concurrency_lock
-          return false unless concurrency_limited?
+          return false unless concurrency_key.present?
 
           Semaphore.signal(self)
         end
diff --git a/app/models/solid_queue/semaphore.rb b/app/models/solid_queue/semaphore.rb
index 7d111ac55..1e0f26462 100644
--- a/app/models/solid_queue/semaphore.rb
+++ b/app/models/solid_queue/semaphore.rb
@@ -85,7 +85,7 @@ def key
         end
 
         def expires_at
-          job.concurrency_duration.from_now
+          (job.concurrency_duration || SolidQueue.default_concurrency_control_period).from_now
         end
 
         def limit
diff --git a/test/models/solid_queue/claimed_execution_concurrency_test.rb b/test/models/solid_queue/claimed_execution_concurrency_test.rb
index 1847d49f3..404416dba 100644
--- a/test/models/solid_queue/claimed_execution_concurrency_test.rb
+++ b/test/models/solid_queue/claimed_execution_concurrency_test.rb
@@ -158,6 +158,23 @@ class SolidQueue::ClaimedExecutionConcurrencyTest < ActiveSupport::TestCase
     end
   end
 
+  test "a job whose class is gone by the time it runs releases its concurrency lock" do
+    first_job, claimed_execution, concurrency_key = enqueue_blocked_group
+    assert_equal 0, SolidQueue::Semaphore.find_by!(key: concurrency_key).value
+
+    # A worker on a release that no longer defines the class claims the job.
+    first_job.update_columns(class_name: "RemovedJobClass")
+    claimed_execution.reload
+
+    assert_raises SolidQueue::Job::ClassMissingError do
+      claimed_execution.perform
+    end
+
+    assert first_job.reload.failed?
+    assert SolidQueue::ReadyExecution.where(job_id: SolidQueue::Job.where(concurrency_key: concurrency_key).select(:id)).exists?,
+      "the failed job should release the next blocked job of its key"
+  end
+
   private
     # Enqueues three concurrency-limited jobs over the same key. The first is
     # claimed; the other two are blocked. Returns the first job, its claimed