diff --git a/lib/liquid/context.rb b/lib/liquid/context.rb
index 94bd4f5..6e8ceb8 100644
--- a/lib/liquid/context.rb
+++ b/lib/liquid/context.rb
@@ -151,7 +151,7 @@ module Liquid
 
     # Only allow String, Numeric, Hash, Array, Proc, Boolean or <tt>Liquid::Drop</tt>
     def []=(key, value)
-      if !@this_stack_used
+      unless @this_stack_used
         @this_stack_used = true
         push({})
       end