diff --git a/lib/zero/response.rb b/lib/zero/response.rb
index 2660924..eec4559 100644
--- a/lib/zero/response.rb
+++ b/lib/zero/response.rb
@@ -32,8 +32,8 @@ module Zero
     #
     def to_a()
       # TODO Remove content length and body, on certain status codes
-      # TODO Set content length, if not already set
-      content_length
+      # Set content length, if not already set
+      content_length unless header.has_key? 'Content-Length'
       # TODO Set content type, if not already set
 
       [status, header, body]
diff --git a/spec/unit/response/response_spec.rb b/spec/unit/response/response_spec.rb
index 9f6a3a0..b6428fc 100644
--- a/spec/unit/response/response_spec.rb
+++ b/spec/unit/response/response_spec.rb
@@ -19,12 +19,19 @@ describe Zero::Response do
     end
 
     it "returns the content length in the header" do
-      subject.body   = ['foobar']
-
+      subject.body = ['foobar']
       value = subject.to_a
 
       value[1].should eq({'Content-Length' => 6})  # Headers
     end
+
+    it "does not fix the Content-Length, if it's already set" do
+      subject.body = ['foobar']
+      subject.header = {'Content-Length' => 3}
+      value = subject.to_a
+
+      value[1].should eq({'Content-Length' => 3})  # Headers
+    end
   end
 
   describe '#status' do