diff --git a/Octokit/Exceptions/ApiException.cs b/Octokit/Exceptions/ApiException.cs
index bbbae6a3..ad3b93d8 100644
--- a/Octokit/Exceptions/ApiException.cs
+++ b/Octokit/Exceptions/ApiException.cs
@@ -192,7 +192,7 @@ namespace Octokit
             get
             {
                 return HttpResponse != null
-                       && !HttpResponse.ContentType.StartsWith("image/")
+                       && !HttpResponse.ContentType.StartsWith("image/", StringComparison.OrdinalIgnoreCase)
                        && HttpResponse.Body is string
                     ? (string)HttpResponse.Body : string.Empty;
             }