"Invalid relationship value. Must be either a document ID or a document, array given.

In my app i am trying to update the documents in my databse from a form in my app, and i am getting this error
Error updating event users: AppwriteException: Invalid relationship value. Must be either a document ID or a document, array given.
at async updateEventUsers (lib/actions/events.actions.ts:138:29)
136 |
137 | // Update the event document

138 | const updatedEvent = await databases.updateDocument(
| ^
139 | appwriteConfig.databaseId,
140 | appwriteConfig.eventCollectionId,
141 | eventId, {
code: 400,
type: 'relationship_value_invalid',
response: '{"message":"Invalid relationship value. Must be either a document ID or a document, array given.","code":400,"type":"relationship_value_invalid","version":"1.6.2"}'
}
⨯ Error: Failed to update event users. Please try again.
at updateEventUsers (lib/actions/events.actions.ts:151:14)
149 | } catch (error) {
150 | console.error("Error updating event users:", error);
151 | throw new Error("Failed to update event users. Please try again.");
| ^
152 | }
153 | }
154 | {
digest: '2401185207'
}

I am tyring to work out if updating relationship attbuties is not something i can do?