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

# What Unique ID Values Does the System Accept?

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.