RequestLibrary : Post On session vs Post request

Hello,
I have some issues with the new requestlibrary version (0.9.1).

Here is my former code (with the 0.7.1 version) which works fine :

${header}= Create Dictionary Content-Type=application/json;charset=utf-8 Accept=application/json;charset=utf-8
Create Session mysession ${base_url} proxies=${proxy} headers=${header}
${body}= Get File MyFile.json
${response}= Post Request mysession myURI data=${body}

With the new version (0.9.1), I only modify the keyword :

${header}= Create Dictionary Content-Type=application/json;charset=utf-8 Accept=application/json;charset=utf-8
Create Session mysession ${base_url} proxies=${proxy} headers=${header}
${body}= Get File MyFile.json
${response}= Post On Session mysession myURI data=${body}

The JSON response is “BAD REQUEST” and I don’t know why… An idea ?

Thanks a lot !