Firebase Set transaction exception - firebase

Question

I am getting this exception while creating a document in Firestore transaction using Flutter cloud_firestore package:
PlatformException(Error performing transaction, Every document read in a transaction must also be written., null)
This is the code:
DocumentSnapshot postSnapshot = await tx.get(post.reference);
await tx.set(Firestore.instance.collection('users').document(userId).collection('stars').
document(),docNew);
await tx.update(postSnapshot.reference, stockUpdate);
Below is the exception details:
E/flutter ( 7157): [ERROR:flutter/lib/ui/ui_dart_state.cc(157)] Unhandled Exception: PlatformException(Error performing transaction, Every document read in a transaction must also be written., null)
E/flutter ( 7157): #0 StandardMethodCodec.decodeEnvelope (package:flutter/src/services/message_codecs.dart:569:7)
E/flutter ( 7157): #1 MethodChannel.invokeMethod (package:flutter/src/services/platform_channel.dart:321:33)
E/flutter ( 7157): <asynchronous suspension>
E/flutter ( 7157): #2 MethodChannel.invokeMapMethod (package:flutter/src/services/platform_channel.dart:349:48)
E/flutter ( 7157): #3 MethodChannelFirestore.runTransaction (package:cloud_firestore_platform_interface/src/method_channel/method_channel_firestore.dart:123:10)
E/flutter ( 7157): #4 Firestore.runTransaction (package:cloud_firestore/src/firestore.dart:85:22)
As you see in the code, I try to create doc in one collection and update a field in another document.
Any hints to debug this? Thank you.