diff --git a/packages/firestore/src/model/mutation.ts b/packages/firestore/src/model/mutation.ts
index c6eef58aac5..7778fd49eb4 100644
--- a/packages/firestore/src/model/mutation.ts
+++ b/packages/firestore/src/model/mutation.ts
@@ -91,9 +91,9 @@ export class MutationResult {
      * containing field transforms has been committed. Contains one FieldValue
      * for each FieldTransform that was in the mutation.
      *
-     * Will be null if the mutation did not contain any field transforms.
+     * Will be empty if the mutation did not contain any field transforms.
      */
-    readonly transformResults: Array<ProtoValue | null> | null
+    readonly transformResults: Array<ProtoValue | null>
   ) {}
 }
 
@@ -434,18 +434,16 @@ function applySetMutationToRemoteDocument(
   // remote document the server has accepted the mutation so the precondition
   // must have held.
   let newData = mutation.value;
-  if (mutationResult.transformResults) {
-    const transformResults = serverTransformResults(
-      mutation.fieldTransforms,
-      maybeDoc,
-      mutationResult.transformResults
-    );
-    newData = transformObject(
-      mutation.fieldTransforms,
-      newData,
-      transformResults
-    );
-  }
+  const transformResults = serverTransformResults(
+    mutation.fieldTransforms,
+    maybeDoc,
+    mutationResult.transformResults
+  );
+  newData = transformObject(
+    mutation.fieldTransforms,
+    newData,
+    transformResults
+  );
 
   return new Document(mutation.key, mutationResult.version, newData, {
     hasCommittedMutations: true
@@ -519,13 +517,11 @@ function applyPatchMutationToRemoteDocument(
     return new UnknownDocument(mutation.key, mutationResult.version);
   }
 
-  const transformResults = mutationResult.transformResults
-    ? serverTransformResults(
-        mutation.fieldTransforms,
-        maybeDoc,
-        mutationResult.transformResults
-      )
-    : [];
+  const transformResults = serverTransformResults(
+    mutation.fieldTransforms,
+    maybeDoc,
+    mutationResult.transformResults
+  );
   const newData = patchDocument(mutation, maybeDoc, transformResults);
   return new Document(mutation.key, mutationResult.version, newData, {
     hasCommittedMutations: true
@@ -701,7 +697,7 @@ function applyDeleteMutationToRemoteDocument(
   mutationResult: MutationResult
 ): NoDocument {
   debugAssert(
-    mutationResult.transformResults == null,
+    mutationResult.transformResults.length === 0,
     'Transform results received by DeleteMutation.'
   );
 
diff --git a/packages/firestore/src/remote/serializer.ts b/packages/firestore/src/remote/serializer.ts
index 40242d690f3..8fba038547b 100644
--- a/packages/firestore/src/remote/serializer.ts
+++ b/packages/firestore/src/remote/serializer.ts
@@ -81,7 +81,6 @@ import {
   Target as ProtoTarget,
   TargetChangeTargetChangeType as ProtoTargetChangeTargetChangeType,
   Timestamp as ProtoTimestamp,
-  Value as ProtoValue,
   Write as ProtoWrite,
   WriteResult as ProtoWriteResult
 } from '../protos/firestore_proto_api';
@@ -698,11 +697,7 @@ function fromWriteResult(
     version = fromVersion(commitTime);
   }
 
-  let transformResults: ProtoValue[] | null = null;
-  if (proto.transformResults && proto.transformResults.length > 0) {
-    transformResults = proto.transformResults;
-  }
-  return new MutationResult(version, transformResults);
+  return new MutationResult(version, proto.transformResults || []);
 }
 
 export function fromWriteResults(
diff --git a/packages/firestore/test/unit/local/local_store.test.ts b/packages/firestore/test/unit/local/local_store.test.ts
index 6a68e9ca7a6..bd67b579c57 100644
--- a/packages/firestore/test/unit/local/local_store.test.ts
+++ b/packages/firestore/test/unit/local/local_store.test.ts
@@ -255,7 +255,7 @@ class LocalStoreTester {
         const mutationResults = [
           new MutationResult(
             ver,
-            options.transformResult ? [options.transformResult] : null
+            options.transformResult ? [options.transformResult] : []
           )
         ];
         const write = MutationBatchResult.from(batch, ver, mutationResults);
diff --git a/packages/firestore/test/unit/model/mutation.test.ts b/packages/firestore/test/unit/model/mutation.test.ts
index eacf47df2d5..1083b0b63be 100644
--- a/packages/firestore/test/unit/model/mutation.test.ts
+++ b/packages/firestore/test/unit/model/mutation.test.ts
@@ -582,7 +582,7 @@ describe('Mutation', () => {
 
     const mutationResult = new MutationResult(
       version(7),
-      /*transformResults=*/ null
+      /*transformResults=*/ []
     );
     const docV7Unknown = unknownDoc('collection/key', 7);
     const docV7Deleted = deletedDoc('collection/key', 7, {
diff --git a/packages/firestore/test/util/helpers.ts b/packages/firestore/test/util/helpers.ts
index 0af124b2a84..ced9a68904d 100644
--- a/packages/firestore/test/util/helpers.ts
+++ b/packages/firestore/test/util/helpers.ts
@@ -284,7 +284,7 @@ export function deleteMutation(keyStr: string): DeleteMutation {
 export function mutationResult(
   testVersion: TestSnapshotVersion
 ): MutationResult {
-  return new MutationResult(version(testVersion), /* transformResults= */ null);
+  return new MutationResult(version(testVersion), /* transformResults= */ []);
 }
 
 export function bound(