Steps to Reproduce:

1. Create a new User Picker(Multiple User) Custom Field and assign in on Service Desk screen.
2. Edit on of the Request Types fields and add the Custom Field.
3. Create a new User with a Space e.g. "customer 1"
4. Go to Customer Portal and create a request with a Request Type that have the Custom Field.
5. Include the "Customer 1" on the field and create the request.

Expected Result:
The request will be created

Actual Result:
The request is not created and there is an error for the User Picker(Multiple User) Custom Field as seen below.