There is a lot of changes in customize like this: - (error "Keyword %s is missing an argument" keyword)) + (signal 'error (list "Keyword is missing an argument" keyword))) Presumably dating back to this: 1997-12-15 Hrvoje Niksic <hniksic(a)srce.hr&gt; * widget.el (define-widget): Check the arguments. * cus-edit.el (customize-face): Use `check-argument-type'. (custom-variable-value-create): Use `signal' to signal error. (custom-variable-reset-saved): Ditto. (custom-variable-reset-standard): Ditto. (custom-face-reset-saved): Ditto. ... What is the purpose of that change?