diff --git a/VERSION b/VERSION
index bcab45a..81340c7 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-0.0.3
+0.0.4
diff --git a/lib/fluent/plugin/filter_flatten.rb b/lib/fluent/plugin/filter_flatten.rb
index 69100ab..96819c8 100644
--- a/lib/fluent/plugin/filter_flatten.rb
+++ b/lib/fluent/plugin/filter_flatten.rb
@@ -40,10 +40,18 @@ def filter(tag, time, record)
     end
 
     def flatten(record)
+      unless @field.to_s.length > 0
+        raise Fluent::ConfigError, "Invalid invocation: field not defined"
+      end
+
       newrecord = {}
 
       record.each do |key, value|
-        newkey = key.gsub(/\./, @separator)
+        if key.match(@field)
+          newkey = key.gsub(/\./, @separator)
+        else
+          newkey = key
+        end
 
         # Recurse hashes and arrays:
         if @recurse