diff --git a/lib/sinatra/base.rb b/lib/sinatra/base.rb
index 0e72e1a1..179b7d0e 100644
--- a/lib/sinatra/base.rb
+++ b/lib/sinatra/base.rb
@@ -990,7 +990,7 @@ module Sinatra
       # Will set params[:agent].
       def user_agent(pattern)
         condition do
-          if request.user_agent =~ pattern
+          if request.user_agent.to_s =~ pattern
             @params[:agent] = $~[1..-1]
             true
           else
diff --git a/test/routing_test.rb b/test/routing_test.rb
index 795de34c..0da34306 100644
--- a/test/routing_test.rb
+++ b/test/routing_test.rb
@@ -611,6 +611,18 @@ class RoutingTest < Test::Unit::TestCase
     assert_equal 'Hello World', body
   end
 
+  it "treats missing user agent like an empty string" do
+    mock_app do
+      user_agent(/.*/)
+      get '/' do
+        "Hello World"
+      end
+    end
+    get '/'
+    assert_equal 200, status
+    assert_equal 'Hello World', body
+  end
+
   it "makes captures in user agent pattern available in params[:agent]" do
     mock_app {
       user_agent(/Foo (.*)/)