diff --git a/packages/firestore/lib/utils/index.js b/packages/firestore/lib/utils/index.js
index 8c7aba68..f6c89a5f 100644
--- a/packages/firestore/lib/utils/index.js
+++ b/packages/firestore/lib/utils/index.js
@@ -58,7 +58,7 @@ export function parseUpdateArgs(args) {
       if (isString(key)) {
         data[key] = value;
       } else if (key instanceof FirestoreFieldPath) {
-        data = mergeFieldPathData(data, key._segments, value);
+        data[key._toPath()] = value;
       } else {
         throw new Error(`argument at index ${i} must be a string or FieldPath`);
       }
@@ -141,25 +141,6 @@ function buildFieldPathData(segments, value) {
   };
 }
 
-export function mergeFieldPathData(data, segments, value) {
-  if (segments.length === 1) {
-    return {
-      ...data,
-      [segments[0]]: value,
-    };
-  }
-  if (data[segments[0]]) {
-    return {
-      ...data,
-      [segments[0]]: mergeFieldPathData(data[segments[0]], segments.slice(1), value),
-    };
-  }
-  return {
-    ...data,
-    [segments[0]]: buildFieldPathData(segments.slice(1), value),
-  };
-}
-
 export function parseSnapshotArgs(args) {
   if (args.length === 0) {
     throw new Error('expected at least one argument.');