diff --git a/lib/rackstash/message.rb b/lib/rackstash/message.rb
index 2d788e6..a26d877 100644
--- a/lib/rackstash/message.rb
+++ b/lib/rackstash/message.rb
@@ -42,18 +42,19 @@ module Rackstash
       @time = dup_freeze(time)
       @progname = dup_freeze(progname)
       @formatter = formatter
+
+      # Freeze the newly created message to ensure it can't be changed.
+      # All passed values are also effectively frozen, making the Message an
+      # immutable object.
+      freeze
     end
 
     def severity_label
       SEVERITY_LABEL[@severity] || SEVERITY_LABEL.last
     end
 
-    def frozen?
-      true
-    end
-
     def to_s
-      @to_s ||= @formatter.call(severity_label, @time, @progname, @message)
+      @formatter.call(severity_label, @time, @progname, @message)
     end
     alias_method :to_str, :to_s
     alias_method :as_json, :to_s
diff --git a/spec/rackstash/message_spec.rb b/spec/rackstash/message_spec.rb
index 4563548..0a0db5f 100644
--- a/spec/rackstash/message_spec.rb
+++ b/spec/rackstash/message_spec.rb
@@ -139,7 +139,7 @@ describe Rackstash::Message do
       message = 'Hello World'
 
       formatter = double('formatter')
-      expect(formatter).to receive(:call).once
+      expect(formatter).to receive(:call)
         .with('DEBUG', time, progname, message)
         .and_return('Formatted Message')
 
@@ -152,8 +152,12 @@ describe Rackstash::Message do
       )
 
       expect(message.to_s).to eql 'Formatted Message'
-      # Same result, but no additional call to the formatter
-      expect(message.to_s).to eql 'Formatted Message'
+    end
+  end
+
+  describe '#frozen?' do
+    it 'is always true' do
+      expect(Rackstash::Message.new('Beep boop')).to be_frozen
     end
   end
 end