Why am I getting an external ID error when I try to merge this user?

Disclaimer: This article is provided for instructional purposes only. Zendesk does not support or guarantee the code. Please post any issues you have in the comments section or try searching for a solution online.  

Question 

Agents are unable to merge end-users together. Each time they try, they are met with the following error message:

You cannot merge [user] while they have an external ID.
Please close this window and try again. 

Answer

External IDs are typically used by an SSO system to identify users by something other than an email address. It is because of this that end-users with external IDs can not be merged together. In order to do this, the external ID of the user being merged would have to be removed or essentially replaced with a null value. 

You can use an API request similar to this to remove the external ID:

curl -u {your_email}:{password} https://{subdomain}.zendesk.com/api/v2/users/{userID}.json -H "Content-Type: application/json" -X PUT -d '{"user": {"external_id":""}}'

Once removed, agents should be able to merge the user identities.