diff --git a/java/google/registry/flows/domain/DomainCreateFlow.java b/java/google/registry/flows/domain/DomainCreateFlow.java
index 36917f60b..a8b0e0cab 100644
--- a/java/google/registry/flows/domain/DomainCreateFlow.java
+++ b/java/google/registry/flows/domain/DomainCreateFlow.java
@@ -157,7 +157,9 @@ public class DomainCreateFlow extends DomainCreateOrAllocateFlow {
         .setBillingTime(now.plus(isAnchorTenant()
             ? registry.getAnchorTenantAddGracePeriodLength()
             : registry.getAddGracePeriodLength()))
-        .setFlags(isAnchorTenant() ? ImmutableSet.of(BillingEvent.Flag.ANCHOR_TENANT) : null)
+        .setFlags(isAnchorTenant()
+            ? ImmutableSet.of(BillingEvent.Flag.ANCHOR_TENANT)
+            : ImmutableSet.<BillingEvent.Flag>of())
         .setParent(historyEntry)
         .build();
     ofy().save().entity(createEvent);
diff --git a/java/google/registry/model/billing/BillingEvent.java b/java/google/registry/model/billing/BillingEvent.java
index 3a1337172..bc6d430eb 100644
--- a/java/google/registry/model/billing/BillingEvent.java
+++ b/java/google/registry/model/billing/BillingEvent.java
@@ -19,9 +19,11 @@ import static com.google.common.base.Preconditions.checkArgument;
 import static com.google.common.base.Preconditions.checkNotNull;
 import static com.google.common.base.Preconditions.checkState;
 import static google.registry.model.ofy.ObjectifyService.ofy;
+import static google.registry.util.CollectionUtils.forceEmptyToNull;
 import static google.registry.util.CollectionUtils.nullToEmptyImmutableCopy;
 import static google.registry.util.CollectionUtils.union;
 import static google.registry.util.DateTimeUtils.END_OF_TIME;
+import static google.registry.util.PreconditionsUtils.checkArgumentNotNull;
 
 import com.google.common.base.Optional;
 import com.google.common.collect.ImmutableMap;
@@ -44,6 +46,7 @@ import google.registry.model.reporting.HistoryEntry;
 import google.registry.model.transfer.TransferData.TransferServerApproveEntity;
 import java.util.Objects;
 import java.util.Set;
+import javax.annotation.Nullable;
 import org.joda.money.Money;
 import org.joda.time.DateTime;
 
@@ -98,6 +101,7 @@ public abstract class BillingEvent extends ImmutableObject
   /** The fully qualified domain name of the domain that the bill is for. */
   String targetId;
 
+  @Nullable
   Set<Flag> flags;
 
   public String getClientId() {
@@ -168,7 +172,7 @@ public abstract class BillingEvent extends ImmutableObject
     }
 
     public B setFlags(ImmutableSet<Flag> flags) {
-      getInstance().flags = flags;
+      getInstance().flags = forceEmptyToNull(checkArgumentNotNull(flags, "flags"));
       return thisCastToDerived();
     }
 
diff --git a/javatests/google/registry/flows/domain/DomainCreateFlowTest.java b/javatests/google/registry/flows/domain/DomainCreateFlowTest.java
index 14580ad1b..202ce5b80 100644
--- a/javatests/google/registry/flows/domain/DomainCreateFlowTest.java
+++ b/javatests/google/registry/flows/domain/DomainCreateFlowTest.java
@@ -178,7 +178,7 @@ public class DomainCreateFlowTest extends ResourceFlowTestCase<DomainCreateFlow,
         : clock.nowUtc().plus(Registry.get(domainTld).getAddGracePeriodLength());
     ImmutableSet<BillingEvent.Flag> billingFlags = isAnchorTenant
         ? ImmutableSet.of(BillingEvent.Flag.ANCHOR_TENANT)
-        : null;
+        : ImmutableSet.<BillingEvent.Flag>of();
     HistoryEntry historyEntry = getHistoryEntries(domain).get(0);
     assertAboutDomains().that(domain)
         .hasRegistrationExpirationTime(