diff --git a/appveyor.yml b/appveyor.yml
index cdfb810..f247694 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -1,4 +1,4 @@
-version: 0.9.{build}
+version: 1.0.{build}
 image: Visual Studio 2017
 configuration: Release
 skip_tags: true
diff --git a/src/GoCardless.Api/Core/Configuration/ClientConfiguration.cs b/src/GoCardless.Api/Core/Configuration/ClientConfiguration.cs
index 7954a2c..d1b0e47 100644
--- a/src/GoCardless.Api/Core/Configuration/ClientConfiguration.cs
+++ b/src/GoCardless.Api/Core/Configuration/ClientConfiguration.cs
@@ -22,12 +22,12 @@ private ClientConfiguration(bool live, string accessToken)
                 throw new ArgumentException("Value is null, empty or whitespace.", nameof(accessToken));
             }
 
+            AccessToken = accessToken;
+
             BaseUri = live
                 ? "https://api.gocardless.com/"
                 : "https://api-sandbox.gocardless.com/";
 
-            AccessToken = accessToken;
-
             Headers = new Dictionary<string, string>
             {
                 ["Authorization"] = $"Bearer {AccessToken}",
@@ -37,6 +37,6 @@ private ClientConfiguration(bool live, string accessToken)
         
         public string BaseUri { get; }
         public string AccessToken { get; }
-        public IReadOnlyDictionary<string, string> Headers { get; }
+        public IDictionary<string, string> Headers { get; }
     }
 }
\ No newline at end of file