diff --git a/lib/coveralls/api.rb b/lib/coveralls/api.rb
index bc79c09e..f0be7bbf 100644
--- a/lib/coveralls/api.rb
+++ b/lib/coveralls/api.rb
@@ -53,8 +53,8 @@ def self.endpoint_to_url(endpoint)
 
     def self.hash_to_file(hash)
       file = nil
-      Tempfile.open(['coveralls-upload', '.json']) do |f|
-        f.write(MultiJson.dump hash)
+      Tempfile.open(['coveralls-upload', 'json']) do |f|
+        f.write(JSON.dump hash)
         file = f
       end
       File.new(file.path, 'rb')