diff --git a/app/decorators/asciicast_decorator.rb b/app/decorators/asciicast_decorator.rb
index def5c59..44791c3 100644
--- a/app/decorators/asciicast_decorator.rb
+++ b/app/decorators/asciicast_decorator.rb
@@ -53,6 +53,8 @@ class AsciicastDecorator < ApplicationDecorator
   end
 
   def os
+    return 'unknown' if uname.blank?
+
     if uname =~ /Linux/
       'Linux'
     elsif uname =~ /Darwin/
diff --git a/spec/decorators/asciicast_decorator_spec.rb b/spec/decorators/asciicast_decorator_spec.rb
index ba530fc..ca82485 100644
--- a/spec/decorators/asciicast_decorator_spec.rb
+++ b/spec/decorators/asciicast_decorator_spec.rb
@@ -2,4 +2,13 @@ require 'spec_helper'
 
 describe AsciicastDecorator do
   before { ApplicationController.new.set_current_view_context }
+
+  describe '#os' do
+    it 'returns "unknown" when uname is blank' do
+      asciicast = Asciicast.new
+      asciicast.uname = nil
+      decorated_asciicast = AsciicastDecorator.new(asciicast)
+      decorated_asciicast.os.should == 'unknown'
+    end
+  end
 end