diff --git a/lib/gistory.rb b/lib/gistory.rb
index ec2a40c..93b45b7 100644
--- a/lib/gistory.rb
+++ b/lib/gistory.rb
@@ -3,6 +3,7 @@
 
 require 'gistory/cli/main'
 require 'gistory/cli/arg_parser'
+require 'gistory/cli/io'
 require 'gistory/configuration'
 
 require 'gistory/errors'
diff --git a/lib/gistory/cli/arg_parser.rb b/lib/gistory/cli/arg_parser.rb
index a0a17ea..ec31da1 100644
--- a/lib/gistory/cli/arg_parser.rb
+++ b/lib/gistory/cli/arg_parser.rb
@@ -4,16 +4,18 @@
 module Gistory
   module Cli
     class ArgParser
-      def initialize(args:)
+      def initialize(args:, io: Gistory::Cli::Io.new)
         @args = args
         @config = Gistory.config
         @parser = create_parser(@config)
+        @io = io
       end
 
       def parse
         @parser.parse!(@args)
 
         parse_gem_name
+        @io.error("extra parameters ignored: #{@args}") unless @args.count.zero?
         @config
       rescue OptionParser::InvalidOption => err
         raise(Gistory::ParserError, err.message)
@@ -60,20 +62,21 @@ def add_max_lockfile_changes(parser, config)
         default = config.max_lockfile_changes
         description = "max number of changes to the lock file (default #{default})"
         parser.on('-m', '--max-lockfile-changes [INTEGER]', Integer, description) do |m|
+          raise(Gistory::ParserError, 'argument --max-lockfile-changes must be an integer') if m.nil?
           config.max_lockfile_changes = m
         end
       end
 
       def add_help(parser)
         parser.on_tail('-h', '--help', 'Show this message') do
-          puts parser
+          @io.puts parser
           exit
         end
       end
 
       def add_version(parser)
         parser.on_tail('--version', 'Show version') do
-          puts Gistory::VERSION
+          @io.puts "gistory version #{Gistory::VERSION}"
           exit
         end
       end
diff --git a/lib/gistory/cli/io.rb b/lib/gistory/cli/io.rb
new file mode 100644
index 0000000..6c2653a
--- /dev/null
+++ b/lib/gistory/cli/io.rb
@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+require 'colorize'
+
+module Gistory
+  module Cli
+    class Io
+      def initialize(out: $stdout, err: $stderr)
+        @out = out
+        @err = err
+      end
+
+      def puts(msg)
+        @out.puts(msg)
+      end
+
+      def error(msg)
+        @err.puts(msg.red)
+      end
+    end
+  end
+end
diff --git a/lib/gistory/cli/main.rb b/lib/gistory/cli/main.rb
index ced0070..e1d5ab3 100644
--- a/lib/gistory/cli/main.rb
+++ b/lib/gistory/cli/main.rb
@@ -1,24 +1,24 @@
 # frozen_string_literal: true
-require 'colorize'
 
 module Gistory
   module Cli
     class Main
-      def initialize(repo_path:, args:)
+      def initialize(repo_path:, args:, io: Gistory::Cli::Io.new)
         @repo_path = repo_path
         @args = args
+        @io = io
       end
 
       def run
         repo = GitRepo.new(path: @repo_path)
-        parser = Cli::ArgParser.new(args: @args)
+        parser = Cli::ArgParser.new(args: @args, io: @io)
         config = parser.parse
         history(repo, config.gem_name)
       rescue Gistory::ParserError => error
-        puts error.message.red
-        puts parser
+        @io.error error.message
+        @io.puts parser
       rescue Gistory::Error => error
-        puts error.message.red
+        @io.error error.message
       end
 
       private
@@ -30,13 +30,13 @@ def history(repo, gem_name)
           raise(Gistory::Error, "Gem '#{gem_name}' not found in lock file, maybe a typo?")
         end
 
-        puts "Gem: #{gem_name}"
-        puts "Current version: #{changes.first.version}"
-        puts ''
+        @io.puts "Gem: #{gem_name}"
+        @io.puts "Current version: #{changes.first.version}"
+        @io.puts ''
 
-        puts 'Change history:'
+        @io.puts 'Change history:'
         changes.each do |change|
-          puts "#{change.version} on #{change.date.strftime('%a, %e %b %Y %H:%M %Z')} (commit #{change.short_hash})"
+          @io.puts "#{change.version} on #{change.date.strftime('%a, %e %b %Y %H:%M %Z')} (commit #{change.short_hash})"
         end
       end
     end
diff --git a/test/cli/arg_parser_test.rb b/test/cli/arg_parser_test.rb
index cdacf55..8414664 100644
--- a/test/cli/arg_parser_test.rb
+++ b/test/cli/arg_parser_test.rb
@@ -25,14 +25,21 @@ def test_raises_no_args
       end
 
       def test_raises_no_gem
-        parser = ArgParser.new(args: ['-max-lockfile-changes=10'])
+        parser = ArgParser.new(args: ['--max-lockfile-changes', '10'])
         assert_raises do
           parser.parse
         end
       end
 
       def test_raises_invalid_args
-        parser = ArgParser.new(args: ['mygem', '-max-lockfile-changes=10', '-z1'])
+        parser = ArgParser.new(args: ['mygem', '-m10', '-z1'])
+        assert_raises Gistory::ParserError do
+          parser.parse
+        end
+      end
+
+      def test_raises_max_lockfile_changes_non_integer
+        parser = ArgParser.new(args: ['mygem', '-mA'])
         assert_raises Gistory::ParserError do
           parser.parse
         end