diff --git a/module/react_group.amlg b/module/react_group.amlg
index 954c65a0e..f6b837af0 100644
--- a/module/react_group.amlg
+++ b/module/react_group.amlg
@@ -133,42 +133,46 @@
 
 		(declare (assoc
 			react_results
-				;don't run in parallel since SingleReactGroup creates and destroys entities
-				(map
-					(lambda (seq
-						;update progress every 'progress_update_size' number of reacts
-						(if (= 0 (mod (current_index) progress_update_size))
-							(if (< num_progress_updates max_progress_updates)
-								(seq
-									(accum (assoc num_progress_updates 1))
-									(call !ProgressUpdate (assoc details "Reacting to a group of cases"))
+				;ReactGroup can run in parallel only when working on existing cases. When using new cases,
+				;it must instead run serially since it creates ephemeral entities for each new case.
+				(call (set_concurrency
+					(map
+						(lambda (seq
+							;update progress every 'progress_update_size' number of reacts
+							(if (= 0 (mod (current_index) progress_update_size))
+								(if (< num_progress_updates max_progress_updates)
+									(seq
+										(accum (assoc num_progress_updates 1))
+										(call !ProgressUpdate (assoc details "Reacting to a group of cases"))
+									)
 								)
 							)
-						)
 
-						(call !SingleReactGroup (assoc
-							action_features action_features
-							group_id_features group_id_features
-							features features
-							details details
-							new_cases (get new_cases (current_index 1))
-							condition (get conditions (current_index 1))
-							case_indices (get case_indices (current_index 1))
-							familiarity_conviction_addition familiarity_conviction_addition
-							familiarity_conviction_removal familiarity_conviction_removal
-							kl_divergence_addition kl_divergence_addition
-							kl_divergence_removal kl_divergence_removal
-							p_value_of_addition p_value_of_addition
-							p_value_of_removal p_value_of_removal
-							distance_contributions distance_contributions
-							residual_contributions residual_contributions
-							similarity_conviction similarity_conviction
-							weight_feature weight_feature
-							use_case_weights use_case_weights
+							(call !SingleReactGroup (assoc
+								action_features action_features
+								group_id_features group_id_features
+								features features
+								details details
+								new_cases (get new_cases (current_index 1))
+								condition (get conditions (current_index 1))
+								case_indices (get case_indices (current_index 1))
+								familiarity_conviction_addition familiarity_conviction_addition
+								familiarity_conviction_removal familiarity_conviction_removal
+								kl_divergence_addition kl_divergence_addition
+								kl_divergence_removal kl_divergence_removal
+								p_value_of_addition p_value_of_addition
+								p_value_of_removal p_value_of_removal
+								distance_contributions distance_contributions
+								residual_contributions residual_contributions
+								similarity_conviction similarity_conviction
+								weight_feature weight_feature
+								use_case_weights use_case_weights
+							))
 						))
-					))
-					(or new_cases conditions case_indices)
-				)
+						(or new_cases conditions case_indices)
+					)
+					(= .null new_cases)
+				))
 		))
 
 		;convert react_results to a dict of react key -> lists of values (one per group)