diff --git a/lib/teaspoon/instrumentation.rb b/lib/teaspoon/instrumentation.rb
index 9f5b3720..3d727c80 100644
--- a/lib/teaspoon/instrumentation.rb
+++ b/lib/teaspoon/instrumentation.rb
@@ -35,10 +35,8 @@ def instrumented_response
 
       result = add_instrumentation(asset)
 
-      asset.instance_variable_set(:@source, result)
-      asset.instance_variable_set(:@length, headers["Content-Length"] = result.bytesize.to_s)
-
-      [status, headers, asset]
+      headers["Content-Length"] = result.bytesize.to_s
+      [status, headers, [result]]
     end
 
     protected
diff --git a/spec/teaspoon/instrumentation_spec.rb b/spec/teaspoon/instrumentation_spec.rb
index aab34c73..28233e8a 100644
--- a/spec/teaspoon/instrumentation_spec.rb
+++ b/spec/teaspoon/instrumentation_spec.rb
@@ -8,6 +8,7 @@
 
   let(:asset) { double(source: source, filename: "path/to/instrument.js") }
   let(:source) { "function add(a, b) { return a + b } // ☃ " }
+  let(:instrumented_source) { source + " // instrumented" }
   let(:response) { [200, { "Content-Type" => "application/javascript" }, asset] }
   let(:env) { { "QUERY_STRING" => "instrument=true" } }
 
@@ -21,7 +22,7 @@
       expect(asset).to receive(:clone).and_return(asset)
 
       allow(File).to receive(:open)
-      allow_any_instance_of(subject).to receive(:instrument).and_return(source + " // instrumented")
+      allow_any_instance_of(subject).to receive(:instrument).and_return(instrumented_source)
 
       path = nil
       Dir.mktmpdir { |p| path = p }
@@ -43,7 +44,7 @@
 
     it "replaces the response array with the appropriate information" do
       response = [666, { "Content-Type" => "application/javascript" }, asset]
-      expected = [666, { "Content-Type" => "application/javascript", "Content-Length" => "59" }, asset]
+      expected = [666, { "Content-Type" => "application/javascript", "Content-Length" => "59" }, [instrumented_source]]
 
       expect(subject.add_to(response, env)).to eq(expected)
     end
@@ -124,10 +125,10 @@ def ignore(paths)
 
     it "instruments a file" do
       pending("needs istanbul to be installed") unless Teaspoon::Instrumentation.executable
-      status, headers, asset = subject.add_to(response, "QUERY_STRING" => "instrument=true")
+      status, headers, body = subject.add_to(response, "QUERY_STRING" => "instrument=true")
       expect(status).to eq(200)
       expect(headers).to include("Content-Type" => "application/javascript")
-      expect(asset.source).to match(/var __cov_.+ = \(Function\('return this'\)\)\(\);/)
+      expect(body.first).to match(/var __cov_.+ = \(Function\('return this'\)\)\(\);/)
     end
 
     it "hooks into sprockets" do