1 files changed, 3 insertions, 0 deletions
diff --git a/lib/ruby2d/window.rb b/lib/ruby2d/window.rb
index 9970861..377523b 100644
--- a/lib/ruby2d/window.rb
+++ b/lib/ruby2d/window.rb
@@ -123,6 +123,9 @@ module Ruby2D
     end
 
     def on(event, &proc)
+      unless @events.has_key? event
+        raise Error, "`#{event}` is not a valid event type"
+      end
       event_id = new_event_key
       @events[event][event_id] = proc
       EventDescriptor.new(event, event_id)