diff --git a/lib/logstash/filters/json_encode.rb b/lib/logstash/filters/json_encode.rb
index 4743ec333..0daef3089 100644
--- a/lib/logstash/filters/json_encode.rb
+++ b/lib/logstash/filters/json_encode.rb
@@ -2,36 +2,34 @@ require "logstash/filters/base"
 require "logstash/namespace"
 
 # JSON encode filter. Takes a field and serializes it into JSON
-class LogStash::Filters::JsonEncode < LogStash::Filters::Base
+#
+# If no target is specified, the source field is overwritten with the JSON
+# text.
+#
+# For example, if you have a field named 'foo', and you want to store the
+# JSON encoded string in 'bar', do this:
+#
+#     filter {
+#       json_encode {
+#         source => "foo"
+#         target => "bar"
+#       }
+#     }
+class LogStash::Filters::JSONEncode < LogStash::Filters::Base
 
   config_name "json_encode"
   milestone 2
 
-  # Config for json_encode is:
-  # 
-  # * source => dest
-  #
-  # For example, if you have a field named 'foo', and you want to store the
-  # JSON encoded string in 'bar', do this:
-  #
-  #     filter {
-  #       json_encode {
-  #         foo => bar
-  #       }
-  #     }
-  #
-  # Note: if the "dest" field already exists, it will be overridden.
-  config /[A-Za-z0-9_@-]+/, :validate => :string
+  # The field to convert to JSON.
+  config :source, :validate => :string, :required => true
+
+  # The field to write the JSON into. If not specified, the source
+  # field will be overwritten.
+  config :target, :validate => :string
 
   public
   def register
-    @json = {}
-
-    @config.each do |field, dest|
-      next if RESERVED.member?(field)
-
-      @json[field] = dest
-    end
+    @target = @source if @target.nil?
   end # def register
 
   public
@@ -40,20 +38,14 @@ class LogStash::Filters::JsonEncode < LogStash::Filters::Base
 
     @logger.debug("Running JSON encoder", :event => event)
 
-    @json.each do |key, dest|
-      next unless event[key]
-
-      begin
-        event[dest] = JSON.pretty_generate(event[key])
-        filter_matched(event)
-      rescue => e
-        event.tag "_jsongeneratefailure"
-        @logger.warn("Trouble encoding JSON", :key => key, :raw => event[key],
-                      :exception => e)
-        next
-      end
+    begin
+      event[@target] = JSON.pretty_generate(event[@source])
+      filter_matched(event)
+    rescue => e
+      event.tag "_jsongeneratefailure"
+      @logger.warn("Trouble encoding JSON", :key => key, :raw => event[key].inspect, :exception => e)
     end
 
-    @logger.debug("Event after JSON encoder", :event => event)
+    @logger.debug? && @logger.debug("Event after JSON encoder", :event => event)
   end # def filter
-end # class LogStash::Filters::JsonEncode
+end # class LogStash::Filters::JSONEncode
diff --git a/spec/filters/json_encode.rb b/spec/filters/json_encode.rb
new file mode 100644
index 000000000..93ccc039e
--- /dev/null
+++ b/spec/filters/json_encode.rb
@@ -0,0 +1,37 @@
+require "test_utils"
+require "logstash/filters/json_encode"
+
+describe LogStash::Filters::JSONEncode do
+  extend LogStash::RSpec
+
+  describe "encode a field as json" do
+    config <<-CONFIG
+      filter {
+        json_encode {
+          source => "hello"
+          target => "fancy"
+        }
+      }
+    CONFIG
+
+    hash = { "hello" => { "whoa" => [ 1,2,3 ] } }
+    sample(hash) do
+      insist { JSON.parse(subject["fancy"]).to_json } == hash["hello"].to_json
+    end
+  end
+
+  describe "encode a field as json and overwrite the original" do
+    config <<-CONFIG
+      filter {
+        json_encode {
+          source => "hello"
+        }
+      }
+    CONFIG
+
+    hash = { "hello" => { "whoa" => [ 1,2,3 ] } }
+    sample(hash) do
+      insist { JSON.parse(subject["hello"]).to_json } == hash["hello"].to_json
+    end
+  end
+end