diff --git a/Octokit/Http/HttpClientAdapter.cs b/Octokit/Http/HttpClientAdapter.cs
index 8f4f6d8345..846beaa056 100644
--- a/Octokit/Http/HttpClientAdapter.cs
+++ b/Octokit/Http/HttpClientAdapter.cs
@@ -109,7 +109,15 @@ protected virtual HttpRequestMessage BuildRequestMessage(IRequest request)
             HttpRequestMessage requestMessage = null;
             try
             {
-                var fullUri = new Uri(request.BaseAddress, request.Endpoint);
+                // Remove any leading slash from endpoint Uri (otherwise it clobbers the /api/v3/ in BaseAddress)
+                var endpoint = request.Endpoint;
+                if (!endpoint.IsAbsoluteUri && 
+                    endpoint.ToString().StartsWith("/", StringComparison.OrdinalIgnoreCase))
+                {
+                    endpoint = new Uri(endpoint.ToString().TrimStart('/'), UriKind.Relative);
+                }
+
+                var fullUri = new Uri(request.BaseAddress, endpoint);
                 requestMessage = new HttpRequestMessage(request.Method, fullUri);
 
                 foreach (var header in request.Headers)