> ## Documentation Index
> Fetch the complete documentation index at: https://moengage.com/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# What Unique ID Values Does the System Accept?

> Know which unique ID values MoEngage accepts and which are restricted. Set up user identifiers correctly to avoid issues during SDK implementation.

This article outlines the key points to consider when setting up a unique identifier (UniqueID) for users in MoEngage.

## Instructions

1. The following values are not allowed in the UniqueID field:
   <CodeGroup>
     ```text Values theme={null}
     "unknown"
     "guest"
     "null"
     "0"
     "1"
     "true"
     "false"
     "user\_attribute\_unique\_id"
     "(empty)"
     “na"
     "n/a"
     ""
     "dummy\_seller\_code"
     "user\_id"
     "id"
     "customer\_id"
     "uid"
     "userid"
     "none"
     "-2"
     "-1"
     "2"
     ```
   </CodeGroup>

2. If you encounter any issues during implementation, please contact your MoEngage Customer Success Manager (CSM) or the Support team.