diff --git a/Octokit.Tests/Http/ConnectionTests.cs b/Octokit.Tests/Http/ConnectionTests.cs
index 30747fdf..4e9de6f6 100644
--- a/Octokit.Tests/Http/ConnectionTests.cs
+++ b/Octokit.Tests/Http/ConnectionTests.cs
@@ -111,9 +111,9 @@ namespace Octokit.Tests.Http
                 string headerKey,
                 string otpHeaderValue)
             {
+                var headers = new Dictionary<string, string> { { headerKey, otpHeaderValue } };
                 var httpClient = Substitute.For<IHttpClient>();
-                IResponse response = new Response { StatusCode = HttpStatusCode.Unauthorized };
-                response.Headers[headerKey] = otpHeaderValue;
+                IResponse response = new Response(HttpStatusCode.Unauthorized, null, headers, "application/json");
                 httpClient.Send(Args.Request, Args.CancellationToken).Returns(Task.FromResult(response));
                 var connection = new Connection(new ProductHeaderValue("OctokitTests"),
                     _exampleUri,
@@ -138,12 +138,9 @@ namespace Octokit.Tests.Http
                 string otpHeaderValue,
                 TwoFactorType expectedFactorType)
             {
+                var headers = new Dictionary<string, string> { { headerKey, otpHeaderValue } };
+                IResponse response = new Response(HttpStatusCode.Unauthorized, null, headers, "application/json");
                 var httpClient = Substitute.For<IHttpClient>();
-                IResponse response = new Response
-                {
-                    StatusCode = HttpStatusCode.Unauthorized,
-                };
-                response.Headers[headerKey] = otpHeaderValue;
                 httpClient.Send(Args.Request, Args.CancellationToken).Returns(Task.FromResult(response));
                 var connection = new Connection(new ProductHeaderValue("OctokitTests"),
                     _exampleUri,
diff --git a/Octokit/Http/ApiResponse.cs b/Octokit/Http/ApiResponse.cs
index e68f8a6f..07252b64 100644
--- a/Octokit/Http/ApiResponse.cs
+++ b/Octokit/Http/ApiResponse.cs
@@ -1,6 +1,4 @@
-using System;
-using System.Collections.Generic;
-using System.Collections.ObjectModel;
+using System.Collections.Generic;
 using System.Net;
 
 namespace Octokit.Internal
@@ -17,7 +15,7 @@ namespace Octokit.Internal
 
             var body = response.Body is T ? (T)response.Body : default(T);
             HttpResponse = response;
-            Headers = new ReadOnlyDictionary<string, string>(response.Headers);
+            Headers = response.Headers;
             Body = body;
             ApiInfo = response.ApiInfo;
             StatusCode = response.StatusCode;
diff --git a/Octokit/Http/IResponse.cs b/Octokit/Http/IResponse.cs
index 1cb48260..0d885e36 100644
--- a/Octokit/Http/IResponse.cs
+++ b/Octokit/Http/IResponse.cs
@@ -18,7 +18,7 @@ namespace Octokit
     public interface IResponse
     {
         object Body { get; }
-        IDictionary<string, string> Headers { get; }
+        IReadOnlyDictionary<string, string> Headers { get; }
         ApiInfo ApiInfo { get; }
         HttpStatusCode StatusCode { get; }
         string ContentType { get; }
diff --git a/Octokit/Http/Response.cs b/Octokit/Http/Response.cs
index 0d01ce79..919f13ca 100644
--- a/Octokit/Http/Response.cs
+++ b/Octokit/Http/Response.cs
@@ -1,8 +1,6 @@
-using System;
-using System.Collections.Generic;
-using System.Linq;
+using System.Collections.Generic;
+using System.Collections.ObjectModel;
 using System.Net;
-using System.Net.Http;
 using Octokit.Internal;
 
 namespace Octokit.Internal
@@ -17,7 +15,7 @@ namespace Octokit.Internal
         {
             Ensure.ArgumentNotNull(headers, "headers");
 
-            Headers = headers;
+            Headers = new ReadOnlyDictionary<string, string>(headers);
             ApiInfo = ApiInfoParser.ParseResponseHeaders(headers);
         }
 
@@ -27,13 +25,13 @@ namespace Octokit.Internal
 
             StatusCode = statusCode;
             Body = body;
-            Headers = headers;
+            Headers = new ReadOnlyDictionary<string, string>(headers);
             ApiInfo = ApiInfoParser.ParseResponseHeaders(headers);
             ContentType = contentType;
         }
 
         public object Body { get; set; }
-        public IDictionary<string, string> Headers { get; private set; }
+        public IReadOnlyDictionary<string, string> Headers { get; private set; }
         public ApiInfo ApiInfo { get; private set; }
         public HttpStatusCode StatusCode { get; set; }
         public string ContentType { get; private set; }