diff --git a/hapi-fhir-jpaserver-base/src/main/java/ca/uhn/fhir/jpa/term/BaseHapiTerminologySvcImpl.java b/hapi-fhir-jpaserver-base/src/main/java/ca/uhn/fhir/jpa/term/BaseHapiTerminologySvcImpl.java
index 0887a04f6bd..fe562f4298c 100644
--- a/hapi-fhir-jpaserver-base/src/main/java/ca/uhn/fhir/jpa/term/BaseHapiTerminologySvcImpl.java
+++ b/hapi-fhir-jpaserver-base/src/main/java/ca/uhn/fhir/jpa/term/BaseHapiTerminologySvcImpl.java
@@ -32,9 +32,11 @@ import ca.uhn.fhir.jpa.util.ScrollableResultsIterator;
 import ca.uhn.fhir.rest.api.server.RequestDetails;
 import ca.uhn.fhir.rest.server.exceptions.InternalErrorException;
 import ca.uhn.fhir.rest.server.exceptions.InvalidRequestException;
+import ca.uhn.fhir.rest.server.exceptions.ResourceNotFoundException;
 import ca.uhn.fhir.rest.server.exceptions.UnprocessableEntityException;
 import ca.uhn.fhir.util.ObjectUtil;
 import ca.uhn.fhir.util.StopWatch;
+import ca.uhn.fhir.util.UrlUtil;
 import ca.uhn.fhir.util.ValidateUtil;
 import com.github.benmanes.caffeine.cache.Cache;
 import com.github.benmanes.caffeine.cache.Caffeine;
@@ -487,7 +489,6 @@ public abstract class BaseHapiTerminologySvcImpl implements IHapiTerminologySvc,
 
 		if (!optionalTermValueSet.isPresent()) {
 			ourLog.warn("ValueSet is not present in terminology tables. Will perform in-memory expansion without parameters. Will schedule this ValueSet for pre-expansion. {}", getValueSetInfo(theValueSetToExpand));
-			myDeferredValueSets.add(theValueSetToExpand);
 			return expandValueSet(theValueSetToExpand); // In-memory expansion.
 		}
 
@@ -2366,6 +2367,10 @@ public abstract class BaseHapiTerminologySvcImpl implements IHapiTerminologySvc,
 		return retVal;
 	}
 
+	protected void throwInvalidValueSet(String theValueSet) {
+		throw new ResourceNotFoundException("Unknown ValueSet: " + UrlUtil.escapeUrlParam(theValueSet));
+	}
+
 	private static void extractLinksFromConceptAndChildren(TermConcept theConcept, List<TermConceptParentChildLink> theLinks) {
 		theLinks.addAll(theConcept.getParents());
 		for (TermConceptParentChildLink child : theConcept.getChildren()) {
diff --git a/hapi-fhir-jpaserver-base/src/main/java/ca/uhn/fhir/jpa/term/HapiTerminologySvcDstu3.java b/hapi-fhir-jpaserver-base/src/main/java/ca/uhn/fhir/jpa/term/HapiTerminologySvcDstu3.java
index 0070469c00f..e50a2fecb7c 100644
--- a/hapi-fhir-jpaserver-base/src/main/java/ca/uhn/fhir/jpa/term/HapiTerminologySvcDstu3.java
+++ b/hapi-fhir-jpaserver-base/src/main/java/ca/uhn/fhir/jpa/term/HapiTerminologySvcDstu3.java
@@ -212,7 +212,7 @@ public class HapiTerminologySvcDstu3 extends BaseHapiTerminologySvcImpl implemen
 	public List<VersionIndependentConcept> expandValueSet(String theValueSet) {
 		ValueSet vs = myValidationSupport.fetchResource(myContext, ValueSet.class, theValueSet);
 		if (vs == null) {
-			return Collections.emptyList();
+			super.throwInvalidValueSet(theValueSet);
 		}
 
 		org.hl7.fhir.r4.model.ValueSet valueSetToExpandR4;
diff --git a/hapi-fhir-jpaserver-base/src/main/java/ca/uhn/fhir/jpa/term/HapiTerminologySvcR4.java b/hapi-fhir-jpaserver-base/src/main/java/ca/uhn/fhir/jpa/term/HapiTerminologySvcR4.java
index e9498856f14..e0845156c09 100644
--- a/hapi-fhir-jpaserver-base/src/main/java/ca/uhn/fhir/jpa/term/HapiTerminologySvcR4.java
+++ b/hapi-fhir-jpaserver-base/src/main/java/ca/uhn/fhir/jpa/term/HapiTerminologySvcR4.java
@@ -124,7 +124,7 @@ public class HapiTerminologySvcR4 extends BaseHapiTerminologySvcImpl implements
 	public List<VersionIndependentConcept> expandValueSet(String theValueSet) {//FIXME: DM COWABUNGA
 		ValueSet vs = myValidationSupport.fetchResource(myContext, ValueSet.class, theValueSet);
 		if (vs == null) {
-			return Collections.emptyList();
+			super.throwInvalidValueSet(theValueSet);
 		}
 
 		return expandValueSetAndReturnVersionIndependentConcepts(vs);
diff --git a/hapi-fhir-jpaserver-base/src/main/java/ca/uhn/fhir/jpa/term/HapiTerminologySvcR5.java b/hapi-fhir-jpaserver-base/src/main/java/ca/uhn/fhir/jpa/term/HapiTerminologySvcR5.java
index ede8dc78120..a23d7120fa4 100644
--- a/hapi-fhir-jpaserver-base/src/main/java/ca/uhn/fhir/jpa/term/HapiTerminologySvcR5.java
+++ b/hapi-fhir-jpaserver-base/src/main/java/ca/uhn/fhir/jpa/term/HapiTerminologySvcR5.java
@@ -133,7 +133,7 @@ public class HapiTerminologySvcR5 extends BaseHapiTerminologySvcImpl implements
 	public List<VersionIndependentConcept> expandValueSet(String theValueSet) {
 		ValueSet valueSetR5 = myValidationSupport.fetchResource(myContext, ValueSet.class, theValueSet);
 		if (valueSetR5 == null) {
-			return Collections.emptyList();
+			super.throwInvalidValueSet(theValueSet);
 		}
 
 		return expandValueSetAndReturnVersionIndependentConcepts(org.hl7.fhir.convertors.conv40_50.ValueSet.convertValueSet(valueSetR5));