diff --git a/exe/referator b/exe/referator
index 3553108..230b09d 100755
--- a/exe/referator
+++ b/exe/referator
@@ -10,14 +10,6 @@ require 'referator'
 $stdin.sync = true
 $stdout.sync = true
 
-def success(data = nil)
-  puts "OK #{data.to_json}"
-end
-
-def failure(data = nil)
-  puts "ERR #{data.to_json}"
-end
-
 def serialize_err(err)
   {
     class: err.class.name.freeze,
@@ -33,12 +25,16 @@ workdir = File.expand_path(workdir).freeze
 
 machine = Referator::Machine.new workdir
 
-while (line = $stdin.gets)
+while machine.running?
   begin
+    line = $stdin.gets&.strip || 'exit'
+    line = 'null' if line.empty?
+
     command = Referator::Command.parse line
     result = machine.call command
-    success result
+
+    puts "OK #{result.to_json}"
   rescue => e
-    failure serialize_err e
+    puts "ERR #{serialize_err(e).to_json}"
   end
 end
diff --git a/lib/referator/machine.rb b/lib/referator/machine.rb
index 9fa51df..de27e9e 100644
--- a/lib/referator/machine.rb
+++ b/lib/referator/machine.rb
@@ -7,10 +7,14 @@ module Referator
     def initialize(workdir)
       self.workdir = workdir
 
+      @running = true
+
       @config = Config.new self.workdir
       @footnotes = Footnotes.new @config
     end
 
+    def running? = @running
+
     def call(command)
       method_name = "do_#{command.name}"
       raise 'Invalid command' unless respond_to? method_name, true
@@ -33,7 +37,12 @@ module Referator
     ##################
 
     def do_exit(_command)
-      exit
+      @running = false
+      nil
+    end
+
+    def do_null(_command)
+      nil
     end
 
     def do_ping(_command)
diff --git a/test.rb b/test.rb
index 1cbae33..df8050b 100755
--- a/test.rb
+++ b/test.rb
@@ -89,6 +89,13 @@ def cmd(name, data = nil)
   end
 end
 
+########
+# NULL #
+########
+
+cmd('')    { |result| raise unless result.nil? }
+cmd(:NULL) { |result| raise unless result.nil? }
+
 ###################
 # REGISTER_SCRIPT #
 ###################
@@ -324,3 +331,9 @@ cmd(
   raise unless result ==
                "<sup><a href=\"#link-causa-arcana-com\">[4]</a></sup>\n"
 end
+
+########
+# EXIT #
+########
+
+cmd(:EXIT) { |result| raise unless result.nil? }