diff --git a/lib/rackstash/encoder/json.rb b/lib/rackstash/encoder/json.rb
index 24b877b..78f38d9 100644
--- a/lib/rackstash/encoder/json.rb
+++ b/lib/rackstash/encoder/json.rb
@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 #
-# Copyright 2017 Holger Just
+# Copyright 2017 - 2018 Holger Just
 #
 # This software may be modified and distributed under the terms
 # of the MIT license. See the LICENSE.txt file for details.
@@ -28,7 +28,7 @@ module Rackstash
       # @param event [Hash] a log event as produced by the {Flow}
       # @return [String] the event as a single-line JSON string
       def encode(event)
-        normalize_message(event)
+        normalize_message(event) unless event[FIELD_MESSAGE].nil?
         normalize_timestamp(event)
 
         ::JSON.dump(event)
diff --git a/spec/rackstash/encoder/json_spec.rb b/spec/rackstash/encoder/json_spec.rb
index aac29d6..f8aeb67 100644
--- a/spec/rackstash/encoder/json_spec.rb
+++ b/spec/rackstash/encoder/json_spec.rb
@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 #
-# Copyright 2017 Holger Just
+# Copyright 2017 - 2018 Holger Just
 #
 # This software may be modified and distributed under the terms
 # of the MIT license. See the LICENSE.txt file for details.
@@ -22,5 +22,15 @@ describe Rackstash::Encoder::JSON do
       event = { 'message' => "text\nwith\nnewlines" }
       expect(encoder.encode(event)).to eql '{"message":"text\nwith\nnewlines"}'
     end
+
+    it 'passes the message as nil' do
+      event = { 'message' => nil, 'foo' => 'bar' }
+      expect(encoder.encode(event)).to eql '{"message":null,"foo":"bar"}'
+    end
+
+    it 'omits a missing message' do
+      event = { 'foo' => 'bar' }
+      expect(encoder.encode(event)).to eql '{"foo":"bar"}'
+    end
   end
 end