diff --git a/external_library/gem/toxiclibs/simulation/gray_scott_image.rb b/external_library/gem/toxiclibs/simulation/gray_scott_image.rb
index 90a2788..2d8058f 100644
--- a/external_library/gem/toxiclibs/simulation/gray_scott_image.rb
+++ b/external_library/gem/toxiclibs/simulation/gray_scott_image.rb
@@ -37,7 +37,7 @@
 class GrayScottToneImage < Propane::App
   attr_reader :gs, :tone_map, :img
 
-  KEYS = (1..9).map { |i| i }
+  KEYS = (1..9).map { |i| i.to_s }
 
   def setup
     sketch_title 'Gray Scott Image'