You can only assign default_branch when editing a project or when creating a…

doc/api/projects.md

@@ -592,7 +592,6 @@ Parameters:
 | `name` | string | yes | The name of the new project |
 | `path` | string | no | Custom repository name for new project. By default generated based on name |
 | `namespace_id` | integer | no | Namespace for the new project (defaults to the current user's namespace) |
-| `default_branch` | string | no | `master` by default |
 | `description` | string | no | Short project description |
 | `issues_enabled` | boolean | no | Enable issues for this project |
 | `merge_requests_enabled` | boolean | no | Enable merge requests for this project |
@@ -625,6 +624,7 @@ Parameters:
 | `user_id` | integer | yes | The user ID of the project owner |
 | `name` | string | yes | The name of the new project |
 | `path` | string | no | Custom repository name for new project. By default generated based on name |
+| `default_branch` | string | no | `master` by default |
 | `namespace_id` | integer | no | Namespace for the new project (defaults to the current user's namespace) |
 | `description` | string | no | Short project description |
 | `issues_enabled` | boolean | no | Enable issues for this project |