diff --git a/lisp/transient.el b/lisp/transient.el
index 95c6a94..0d292df 100644
--- a/lisp/transient.el
+++ b/lisp/transient.el
@@ -1138,8 +1138,8 @@ commands are aliases for."
       (when (stringp car)
         (setq args (plist-put args :description pop)))
       (while (keywordp car)
-        (let ((key pop)
-              (val pop))
+        (let* ((key pop)
+               (val (if spec pop (error "No value for `%s'" key))))
           (cond ((eq key :class)
                  (setq class (macroexp-quote val)))
                 ((or (symbolp val)