diff --git a/spiffworkflow-backend/src/spiffworkflow_backend/routes/process_instances_controller.py b/spiffworkflow-backend/src/spiffworkflow_backend/routes/process_instances_controller.py
index 252b9264..ec3015dd 100644
--- a/spiffworkflow-backend/src/spiffworkflow_backend/routes/process_instances_controller.py
+++ b/spiffworkflow-backend/src/spiffworkflow_backend/routes/process_instances_controller.py
@@ -55,9 +55,18 @@ from spiffworkflow_backend.services.error_handling_service import ErrorHandlingS
 from spiffworkflow_backend.services.git_service import GitCommandError
 from spiffworkflow_backend.services.git_service import GitService
 from spiffworkflow_backend.services.message_service import MessageService
+from spiffworkflow_backend.services.process_instance_lock_service import (
+    ProcessInstanceLockService,
+)
 from spiffworkflow_backend.services.process_instance_processor import (
     ProcessInstanceProcessor,
 )
+from spiffworkflow_backend.services.process_instance_queue_service import (
+    ProcessInstanceIsAlreadyLockedError,
+)
+from spiffworkflow_backend.services.process_instance_queue_service import (
+    ProcessInstanceIsNotEnqueuedError,
+)
 from spiffworkflow_backend.services.process_instance_queue_service import (
     ProcessInstanceQueueService,
 )
@@ -129,7 +138,11 @@ def process_instance_run(
         try:
             processor.lock_process_instance("Web")
             processor.do_engine_steps(save=True)
-        except ApiError as e:
+        except (
+            ApiError,
+            ProcessInstanceIsNotEnqueuedError,
+            ProcessInstanceIsAlreadyLockedError,
+        ) as e:
             ErrorHandlingService().handle_error(processor, e)
             raise e
         except Exception as e:
@@ -143,7 +156,8 @@ def process_instance_run(
                 task=task,
             ) from e
         finally:
-            processor.unlock_process_instance("Web")
+            if ProcessInstanceLockService.has_lock(process_instance.id):
+                processor.unlock_process_instance("Web")
 
         if not current_app.config["SPIFFWORKFLOW_BACKEND_RUN_BACKGROUND_SCHEDULER"]:
             MessageService.correlate_all_message_instances()
diff --git a/spiffworkflow-backend/src/spiffworkflow_backend/services/process_instance_queue_service.py b/spiffworkflow-backend/src/spiffworkflow_backend/services/process_instance_queue_service.py
index d9f900b2..d75d903f 100644
--- a/spiffworkflow-backend/src/spiffworkflow_backend/services/process_instance_queue_service.py
+++ b/spiffworkflow-backend/src/spiffworkflow_backend/services/process_instance_queue_service.py
@@ -14,6 +14,10 @@ from spiffworkflow_backend.services.process_instance_lock_service import (
 )
 
 
+class ProcessInstanceIsNotEnqueuedError(Exception):
+    pass
+
+
 class ProcessInstanceIsAlreadyLockedError(Exception):
     pass
 
@@ -62,15 +66,20 @@ class ProcessInstanceQueueService:
             db.session.query(ProcessInstanceQueueModel)
             .filter(
                 ProcessInstanceQueueModel.process_instance_id == process_instance.id,
-                ProcessInstanceQueueModel.locked_by == locked_by,
             )
             .first()
         )
 
         if queue_entry is None:
+            raise ProcessInstanceIsNotEnqueuedError(
+                f"{locked_by} cannot lock process instance {process_instance.id}. It"
+                " has not been enqueued."
+            )
+
+        if queue_entry.locked_by != locked_by:
             raise ProcessInstanceIsAlreadyLockedError(
-                f"Cannot lock process instance {process_instance.id}. "
-                "It has already been locked or has not been enqueued."
+                f"{locked_by} cannot lock process instance {process_instance.id}. "
+                f"It has already been locked by {queue_entry.locked_by}."
             )
 
         ProcessInstanceLockService.lock(process_instance.id, queue_entry)