diff --git a/lib/liquid/standardfilters.rb b/lib/liquid/standardfilters.rb
index 651216ac..7072b081 100644
--- a/lib/liquid/standardfilters.rb
+++ b/lib/liquid/standardfilters.rb
@@ -102,12 +102,12 @@ module Liquid
 
     # Replace occurrences of a string with another
     def replace(input, string, replacement = '')
-      input.to_s.gsub(string, replacement)
+      input.to_s.gsub(string, replacement.to_s)
     end
 
     # Replace the first occurrences of a string with another
     def replace_first(input, string, replacement = '')
-      input.to_s.sub(string, replacement)
+      input.to_s.sub(string, replacement.to_s)
     end
 
     # remove a substring