diff --git a/Octokit.Tests/Clients/IssueTimelineClientTests.cs b/Octokit.Tests/Clients/IssueTimelineClientTests.cs
index d041406a01..d877843647 100644
--- a/Octokit.Tests/Clients/IssueTimelineClientTests.cs
+++ b/Octokit.Tests/Clients/IssueTimelineClientTests.cs
@@ -31,7 +31,7 @@ public async Task RequestsTheCorrectUrl()
                 connection.Received().GetAll<TimelineEventInfo>(
                     Arg.Is<Uri>(u => u.ToString() == "repos/fake/repo/issues/42/timeline"),
                     Arg.Any<Dictionary<string, string>>(),
-                    "application/vnd.github.mockingbird-preview,application/vnd.github.starfox-preview",
+                    "application/vnd.github.mockingbird-preview+json,application/vnd.github.starfox-preview",
                     Arg.Any<ApiOptions>());
             }
 
@@ -46,7 +46,7 @@ public async Task RequestsTheCorrectUrlWithApiOptions()
                 connection.Received().GetAll<TimelineEventInfo>(
                     Arg.Is<Uri>(u => u.ToString() == "repos/fake/repo/issues/42/timeline"),
                     Arg.Any<Dictionary<string, string>>(),
-                    "application/vnd.github.mockingbird-preview,application/vnd.github.starfox-preview",
+                    "application/vnd.github.mockingbird-preview+json,application/vnd.github.starfox-preview",
                     Arg.Is<ApiOptions>(ao => ao.PageSize == 30));
             }
 
@@ -61,7 +61,7 @@ public async Task RequestsTheCorrectUrlWithRepositoryId()
                 connection.Received().GetAll<TimelineEventInfo>(
                     Arg.Is<Uri>(u => u.ToString() == "repositories/1/issues/42/timeline"),
                     Arg.Any<Dictionary<string, string>>(),
-                    "application/vnd.github.mockingbird-preview,application/vnd.github.starfox-preview",
+                    "application/vnd.github.mockingbird-preview+json,application/vnd.github.starfox-preview",
                     Arg.Any<ApiOptions>());
             }
 
@@ -76,7 +76,7 @@ public async Task RequestsTheCorrectUrlWithRepositoryIdAndApiOptions()
                 connection.Received().GetAll<TimelineEventInfo>(
                     Arg.Is<Uri>(u => u.ToString() == "repositories/1/issues/42/timeline"),
                     Arg.Any<Dictionary<string, string>>(),
-                    "application/vnd.github.mockingbird-preview,application/vnd.github.starfox-preview",
+                    "application/vnd.github.mockingbird-preview+json,application/vnd.github.starfox-preview",
                     Arg.Is<ApiOptions>(ao => ao.PageSize == 30));
             }
 
diff --git a/Octokit.Tests/Reactive/ObservableIssueTimelineClientTests.cs b/Octokit.Tests/Reactive/ObservableIssueTimelineClientTests.cs
index dcc02ca275..836d9638d5 100644
--- a/Octokit.Tests/Reactive/ObservableIssueTimelineClientTests.cs
+++ b/Octokit.Tests/Reactive/ObservableIssueTimelineClientTests.cs
@@ -37,7 +37,7 @@ public async Task RequestsCorrectUrl()
                     {
                         ApiInfo = new ApiInfo(new Dictionary<string, Uri>(), new List<string>(), new List<string>(), "etag", new RateLimit()),
                     }, result);
-                gitHubClient.Connection.Get<List<TimelineEventInfo>>(Args.Uri, Args.EmptyDictionary, "application/vnd.github.mockingbird-preview")
+                gitHubClient.Connection.Get<List<TimelineEventInfo>>(Args.Uri, Args.EmptyDictionary, "application/vnd.github.mockingbird-preview+json")
                     .Returns(Task.FromResult(response));
 
                 var timelineEvents = await client.GetAllForIssue("fake", "repo", 42).ToList();
@@ -45,7 +45,7 @@ public async Task RequestsCorrectUrl()
                 connection.Received().Get<List<TimelineEventInfo>>(
                     Arg.Is<Uri>(u => u.ToString() == "repos/fake/repo/issues/42/timeline"),
                     Arg.Any<Dictionary<string, string>>(),
-                    "application/vnd.github.mockingbird-preview");
+                    "application/vnd.github.mockingbird-preview+json");
                 Assert.Equal(1, timelineEvents.Count);
             }
 
@@ -63,7 +63,7 @@ public async Task RequestsCorrectUrlWithApiOptions()
                     {
                         ApiInfo = new ApiInfo(new Dictionary<string, Uri>(), new List<string>(), new List<string>(), "etag", new RateLimit()),
                     }, result);
-                gitHubClient.Connection.Get<List<TimelineEventInfo>>(Args.Uri, Arg.Is<Dictionary<string, string>>(d => d.Count == 1), "application/vnd.github.mockingbird-preview")
+                gitHubClient.Connection.Get<List<TimelineEventInfo>>(Args.Uri, Arg.Is<Dictionary<string, string>>(d => d.Count == 1), "application/vnd.github.mockingbird-preview+json")
                     .Returns(Task.FromResult(response));
 
                 var timelineEvents = await client.GetAllForIssue("fake", "repo", 42, new ApiOptions { PageSize = 30 }).ToList();
@@ -71,7 +71,7 @@ public async Task RequestsCorrectUrlWithApiOptions()
                 connection.Received().Get<List<TimelineEventInfo>>(
                     Arg.Is<Uri>(u => u.ToString() == "repos/fake/repo/issues/42/timeline"),
                     Arg.Is<Dictionary<string, string>>(d => d.Count == 1 && d["per_page"] == "30"),
-                    "application/vnd.github.mockingbird-preview");
+                    "application/vnd.github.mockingbird-preview+json");
                 Assert.Equal(1, timelineEvents.Count);
             }
 
@@ -88,7 +88,7 @@ public async Task RequestCorrectUrlWithRepositoryId()
                     {
                         ApiInfo = new ApiInfo(new Dictionary<string, Uri>(), new List<string>(), new List<string>(), "etag", new RateLimit()),
                     }, result);
-                githubClient.Connection.Get<List<TimelineEventInfo>>(Args.Uri, Args.EmptyDictionary, "application/vnd.github.mockingbird-preview")
+                githubClient.Connection.Get<List<TimelineEventInfo>>(Args.Uri, Args.EmptyDictionary, "application/vnd.github.mockingbird-preview+json")
                     .Returns(Task.FromResult(response));
 
                 var timelineEvents = await client.GetAllForIssue(1, 42).ToList();
@@ -96,7 +96,7 @@ public async Task RequestCorrectUrlWithRepositoryId()
                 connection.Received().Get<List<TimelineEventInfo>>(
                     Arg.Is<Uri>(u => u.ToString() == "repositories/1/issues/42/timeline"),
                     Arg.Any<Dictionary<string, string>>(),
-                    "application/vnd.github.mockingbird-preview");
+                    "application/vnd.github.mockingbird-preview+json");
                 Assert.Equal(1, timelineEvents.Count);
             }
 
@@ -113,7 +113,7 @@ public async Task RequestCorrectUrlWithRepositoryIdAndApiOptions()
                     {
                         ApiInfo = new ApiInfo(new Dictionary<string, Uri>(), new List<string>(), new List<string>(), "etag", new RateLimit()),
                     }, result);
-                githubClient.Connection.Get<List<TimelineEventInfo>>(Args.Uri, Arg.Is<Dictionary<string, string>>(d => d.Count == 1), "application/vnd.github.mockingbird-preview")
+                githubClient.Connection.Get<List<TimelineEventInfo>>(Args.Uri, Arg.Is<Dictionary<string, string>>(d => d.Count == 1), "application/vnd.github.mockingbird-preview+json")
                     .Returns(Task.FromResult(response));
 
                 var timelineEvents = await client.GetAllForIssue(1, 42, new ApiOptions { PageSize = 30 }).ToList();
@@ -121,7 +121,7 @@ public async Task RequestCorrectUrlWithRepositoryIdAndApiOptions()
                 connection.Received().Get<List<TimelineEventInfo>>(
                     Arg.Is<Uri>(u => u.ToString() == "repositories/1/issues/42/timeline"),
                     Arg.Is<Dictionary<string, string>>(d => d.Count == 1 && d["per_page"] == "30"),
-                    "application/vnd.github.mockingbird-preview");
+                    "application/vnd.github.mockingbird-preview+json");
                 Assert.Equal(1, timelineEvents.Count);
             }