diff --git a/src/play_clj/core_graphics.clj b/src/play_clj/core_graphics.clj
index bb141e7..edb3195 100644
--- a/src/play_clj/core_graphics.clj
+++ b/src/play_clj/core_graphics.clj
@@ -64,10 +64,8 @@ complicated shapes. If you use `assoc` to set the overall :x and :y of the
            :y 10
            :angle 45)"
   [type & options]
-  (when (seq (clojure.set/intersection #{:begin :end} (set options)))
-    (-> "No need to call :begin or :end, because it's done for you."
-        Throwable.
-        throw))
+  (assert (empty? (clojure.set/intersection #{:begin :end} (set options)))
+          "No need to call :begin or :end, because it's done for you.")
   `(let [entity# ~(if (keyword? type)
                     `(assoc (shape*) :type (shape-type ~type))
                     type)