diff --git a/misc/fuel/fuel-listener.el b/misc/fuel/fuel-listener.el
index d5fec4bf5f..485d97e81f 100644
--- a/misc/fuel/fuel-listener.el
+++ b/misc/fuel/fuel-listener.el
@@ -192,12 +192,15 @@ With prefix, you're teletransported to the listener's buffer."
     (comint-send-string nil "\"Refreshing loaded vocabs...\" write nl flush")
     (comint-send-string nil " refresh-all \"Done!\" write nl flush\n")))
 
-(defun fuel-test-vocab (vocab)
-  "Run the unit tests for the specified vocabulary."
-  (interactive (list (fuel-completion--read-vocab nil (fuel-syntax--current-vocab))))
-  (comint-send-string (fuel-listener--process)
-                      (concat "\"" vocab "\" reload nl flush\n"
-                              "\"" vocab "\" test nl flush\n")))
+(defun fuel-test-vocab (&optional arg)
+  "Run the unit tests for the current vocabulary. With prefix argument, ask for
+the vocabulary name."
+  (interactive "P")
+  (let* ((vocab (or (and (not arg) (fuel-syntax--current-vocab))
+                    (fuel-completion--read-vocab nil))))
+    (comint-send-string (fuel-listener--process)
+                        (concat "\"" vocab "\" reload nl flush\n"
+                                "\"" vocab "\" test nl flush\n"))))
 
 
 ;;; Completion support