diff --git a/src/client/mod.rs b/src/client/mod.rs
index a7f8c84..320efff 100644
--- a/src/client/mod.rs
+++ b/src/client/mod.rs
@@ -112,7 +112,12 @@ impl<P: Provider> Client<P> {
         Ok(uri.serialize())
     }
 
-    fn post_token(&self, body_pairs: Vec<(&str, &str)>) -> Result<Json, ClientError> {
+    fn post_token<'a>(&'a self, mut body_pairs: Vec<(&str, &'a str)>) -> Result<Json, ClientError> {
+        if P::credentials_in_body() {
+            body_pairs.push(("client_id", &self.client_id));
+            body_pairs.push(("client_secret", &self.client_secret));
+        }
+
         let body = form_urlencoded::serialize(body_pairs);
         let auth_header = header::Authorization(
             header::Basic {
@@ -154,11 +159,6 @@ impl<P: Provider> Client<P> {
             body_pairs.push(("redirect_uri", redirect_uri));
         }
 
-        if P::credentials_in_body() {
-            body_pairs.push(("client_id", &self.client_id));
-            body_pairs.push(("client_secret", &self.client_secret));
-        }
-
         let json = try!(self.post_token(body_pairs));
         let token = try!(P::Token::from_response(&json));
         Ok(token)