I covered most types of fields previously but I didn’t cover the clear multi user field problem in SharePoint Online.
Clearing fields
Table of Contents
A while back I wrote a post about clearing fields in Microsoft Flow. For details on how to update fields please look at the previous post.
Clear Multi user fields
I tried all sorts of ways but I kept hit all sorts of failures in my REST API call to SharePoint.
A collection of the errors that I saw:
An unexpected ‘StartObject’ node was found when reading from the JSON reader. A ‘PrimitiveValue’ node was expected.
A node of type ‘PrimitiveValue’ was read from the JSON reader when trying to read the contents of an entity set reference navigation link; however, a ‘StartObject’ or ‘StartArray’ node was expected.
A node of type ‘StartArray’ was read from the JSON reader when trying to read the contents of a resource reference navigation link; however, a ‘StartObject’ node or ‘PrimitiveValue’ node with null value was expected.
It was not until I looked at how to update a multi user field and then combined this with the how to reset a user field that I got to the solution. In my case I’ve got afield called CheckedBy.

The important bit in the above screenshot is the setting the Id to -1 and the StringId part of the field to an empty string.
'CheckedById': { '__metadata': { 'type': 'Collection(Edm.Int32)' }, 'results': [ -1 ] }, 'CheckedByStringId': { '__metadata': { 'type': 'Collection(Edm.String)' }, 'results': [ '' ] }
Ok, this is great, problem solved!