diff --git a/.rubocop.yml b/.rubocop.yml
index 186ab75..cc4b681 100644
--- a/.rubocop.yml
+++ b/.rubocop.yml
@@ -5,6 +5,10 @@ AllCops:
   NewCops: enable
 
 
+Gemspec/DevelopmentDependencies:
+  Enabled: false
+
+
 Layout/FirstArrayElementIndentation:
   EnforcedStyle: consistent
 
diff --git a/lib/omnikassa2/helpers/csv_serializer.rb b/lib/omnikassa2/helpers/csv_serializer.rb
index d795645..c9c5c00 100644
--- a/lib/omnikassa2/helpers/csv_serializer.rb
+++ b/lib/omnikassa2/helpers/csv_serializer.rb
@@ -8,9 +8,8 @@ def initialize(config)
 
     def serialize(object)
       objects = object.is_a?(Array) ? object : [object]
-      parts = []
-      objects.each do |item|
-        parts << extract_fields(item).join(',')
+      parts = objects.map do |item|
+        extract_fields(item).join(',')
       end
       parts.join(',')
     end
diff --git a/lib/omnikassa2/responses/base_response.rb b/lib/omnikassa2/responses/base_response.rb
index 6d2ad46..38c8eea 100644
--- a/lib/omnikassa2/responses/base_response.rb
+++ b/lib/omnikassa2/responses/base_response.rb
@@ -29,7 +29,7 @@ def success?
     def to_s
       value = ''
       value += "Status: #{code}: #{message}\n"
-      value += "Body: #{(body ? body.to_s : 'nil')}"
+      value += "Body: #{body ? body.to_s : 'nil'}"
       value
     end
   end
diff --git a/omnikassa2.gemspec b/omnikassa2.gemspec
index 10a6df7..ff8453d 100644
--- a/omnikassa2.gemspec
+++ b/omnikassa2.gemspec
@@ -15,7 +15,9 @@ Gem::Specification.new do |spec|
   spec.description   = "Omnikassa2 is a gem for Rabobank's Omnikassa 2.0"
   spec.homepage      = 'https://github.com/KentaaNL/omnikassa2'
 
-  spec.files         = `git ls-files -z`.split("\x0").reject do |f|
+  spec.metadata['rubygems_mfa_required'] = 'true'
+
+  spec.files = `git ls-files -z`.split("\x0").reject do |f|
     f.match(%r{^(test|spec|features)/})
   end