Redmine

Environment:
Redmine version 2.5.1.stable
Ruby version 2.0.0-p481 (2014-05-08) [x86_64-linux]
Rails version 3.2.17
Environment production
Database adapter Mysql2

I am trying to update the content of a wiki page via REST API but I always get an error message complaining that I am posting no content: "text field can't be blank".

I am using the following CURL invocation:

curl -X PUT http://MY-API-KEY@redmine.lan/redmine/projects/foo/wiki/bar.xml -d "<?xml version="1.0"?>
<wiki_page>
  <text>Example</text>
  <comments>Typo</comments>
</wiki_page>" 

and I get the following error message:

<?xml version="1.0" encoding="UTF-8"?><errors type="array"><error>Text field can't be blank</error></errors>

The production log shows the following:

Processing by WikiController#update as XML
  Parameters: {"<?xml version"=>"1.0?>\n<wiki_page>\n  <text>Example</text>\n  <comments>Typo</comments>\n</wiki_page>", "project_id"=>"foo", "id"=>"bar"}
  Current user: user1 (id=23)
  Rendered common/error_messages.api.rsb (0.2ms)