Jira Rest API Updating an issue fixVersion give error - jira

Question

I'm trying to use the JIRA Rest API (version 5) to update an Issue "Fix Version" to set it to a specific version. The current issue I update has some already created bad assignee (the person assigned to it no more exist).
Using the normal Jira web interface, I am able to set the "Fix Version" to the desired value, even if the assignee is wrong.
Using the REST Issue PUT, I received an error:
assignee: "User 'username' does not exist."
My REST call look like this (using chrome Advanced Rest Client and a C# implementation):
Doing a PUT http://.../rest/api/2/issue/[issueId]/ Where [issueId] is the Id of my issue.
My payload is:
{
"update" : {
"fixVersions" : [{
"set" : [{
"name" : "Some Version"
}
]
}
]
}
}
Is it normal that I receive an error on the REST call while I can update the Issue from the web interface ?
Is there a way I can update this issue Fix Version by REST without changing the assignee ?

Answer 1

Here is some code to use:
client = JIRA::Client.new({:consumer_key => CONSUMER_KEY, :consumer_secret => CONSUMER_SECRET})
project = client.Project.find('SAMPLEPROJECT')
project.issues.each do |issue|
puts "#{issue.id} - #{issue.summary}"
end
issue.comments.each {|comment| ... }
comment = issue.comments.build
comment.save({'body':'My new comment'})
comment.delete