The field tag name that is passed to the C function pf_nxfield() is not defined in this form. Review the string that is passed; if it is as you intended, inspect the form definition to see what has been changed. A common error is to confuse the tag name, which is used in the screen layout and to the left of the equal sign in the ATTRIBUTES section, with the field name, which is the column name used to the right of the equal sign in the ATTRIBUTES section. The latter is used when referring to fields.