[fix] setting of pen function in canvas

1 files changed, 4 insertions, 3 deletions

diff --git a/src/canvas-language.lisp b/src/canvas-language.lisp
index b77ad02..6583fc8 100644
--- a/src/canvas-language.lisp
+++ b/src/canvas-language.lisp
@@ -27,10 +27,11 @@
 (defun canvas-pen-color (r g b a)
   (setf *current-pen-color* (mapcar #'colfix (list r g b a))))
 
-(defun canvas-pen-color-function (fn)
+(defun canvas-pen-color-function (&optional fn)
   (setf *current-pen-color-function*
-        (lambda (x y)
-          (mapcar #'colfix (funcall fn x y)))))
+        (when fn
+         (lambda (x y)
+           (mapcar #'colfix (funcall fn x y))))))
 
 (defun canvas-pen-width (n)
   (setf *current-pen-width* (round n)))