diff --git a/lib/OAuth2.php b/lib/OAuth2.php
index b28d4c5..4744761 100644
--- a/lib/OAuth2.php
+++ b/lib/OAuth2.php
@@ -1050,7 +1050,7 @@ protected function grantAccessTokenExtension(IOAuth2Client $client, array $input
     protected function getClientCredentials(array $inputData, array $authHeaders)
     {
         // Basic Authentication is used
-        if (!empty($authHeaders['PHP_AUTH_USER'])) {
+        if (!empty($authHeaders['PHP_AUTH_USER'] && empty($inputData['client_id']))) {
             return array($authHeaders['PHP_AUTH_USER'], $authHeaders['PHP_AUTH_PW']);
         } elseif (empty($inputData['client_id'])) { // No credentials were specified
             throw new OAuth2ServerException(Response::HTTP_BAD_REQUEST, self::ERROR_INVALID_CLIENT, 'Client id was not found in the headers or body');