Expression with concatenation of polystring throws "Cannot execute null+null expression"

Object template is attached.
Trying to create user with givenName and familyName both non-empty will throw:

Cannot execute null+null expression in mapping in objectTemplate:c0c010c0-d34d-b33f-f00d-777111111112(Default User Template)(givenName=null; familyName=null; )

Workaround is to use familyName + '' + givenName + iterationToken

Thanks to Anton Operman to report this.