diff --git a/lib/liquid/drop.rb b/lib/liquid/drop.rb
index 2ad33c85..bac1b3a2 100644
--- a/lib/liquid/drop.rb
+++ b/lib/liquid/drop.rb
@@ -55,12 +55,12 @@ module Liquid
     # Check for method existence without invoking respond_to?, which creates symbols
     def self.invokable?(method_name)
       unless @invokable_methods
-        blacklist = Liquid::Drop.public_instance_methods + [:each]
-        if include?(Enumerable)
-          blacklist += Enumerable.public_instance_methods
-          blacklist -= [:sort, :count, :first, :min, :max]
-        end
         # Ruby 1.8 compatibility: call to_s on method names (which are strings in 1.8, but already symbols in 1.9)
+        blacklist = (Liquid::Drop.public_instance_methods + [:each]).map(&:to_s)
+        if include?(Enumerable)
+          blacklist += Enumerable.public_instance_methods.map(&:to_s)
+          blacklist -= [:sort, :count, :first, :min, :max].map(&:to_s)
+        end
         whitelist = [:to_liquid] + (public_instance_methods.map(&:to_s) - blacklist.map(&:to_s))
         @invokable_methods = Set.new(whitelist.map(&:to_s))
       end