diff --git a/org.hl7.fhir.r5/src/main/java/org/hl7/fhir/r5/context/BaseWorkerContext.java b/org.hl7.fhir.r5/src/main/java/org/hl7/fhir/r5/context/BaseWorkerContext.java
index 71142cb85..cbf40a2e4 100644
--- a/org.hl7.fhir.r5/src/main/java/org/hl7/fhir/r5/context/BaseWorkerContext.java
+++ b/org.hl7.fhir.r5/src/main/java/org/hl7/fhir/r5/context/BaseWorkerContext.java
@@ -381,7 +381,9 @@ public abstract class BaseWorkerContext implements IWorkerContext {
       ValueSet result = txClient.expandValueset(vs, p, params);
       res = new ValueSetExpansionOutcome(result).setTxLink(txLog.getLastId());  
     } catch (Exception e) {
-      res = new ValueSetExpansionOutcome(e.getMessage() == null ? e.getClass().getName() : e.getMessage(), TerminologyServiceErrorClass.UNKNOWN).setTxLink(txLog.getLastId());
+      res = new ValueSetExpansionOutcome(e.getMessage() == null ? e.getClass().getName() : e.getMessage(), TerminologyServiceErrorClass.UNKNOWN);
+      if (txLog != null)
+        res.setTxLink(txLog.getLastId());
     }
     txCache.cacheExpansion(cacheToken, res, TerminologyCache.PERMANENT);
     return res;