SQL error on default values

[PLISuser]

When setting #nu_user_name# as default for a field, it gives an SQL error
===USER==========

globeadmin

===PDO MESSAGE===

SQLSTATE[42000]: Syntax error or access violation: 1064 You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'globeadmin' at line 1

===SQL===========

globeadmin

===BACK TRACE====

C:\wamp\www\nubuilder\nuapi.php - line 2911 (nuRunQuery)

C:\wamp\www\nubuilder\nuapi.php - line 2760 (nuGetDefaultValue)

C:\wamp\www\nubuilder\nuapi.php - line 2297 (nuGetObjectText)

C:\wamp\www\nubuilder\nuapi.php - line 1471 (nuGetObjectsForOneRecord)

C:\wamp\www\nubuilder\nuapi.php - line 190 (nuGetEditForm)

What is going wrong?