Error uploading task: problematic input: target_feature

Hi!
I implemented the uploadOMLTask function for the R interface which produces very bizzare error in case the target_feature of the dataset is either class or Class... openml/openml-r#431

Working example:
able to successfully create the task for the given dataset. task.id returned from server.

<oml:task_inputs xmlns:oml="http://openml.org/openml">
  <oml:task_type_id>1</oml:task_type_id>
  <oml:input name="source_data">41158</oml:input>
  <oml:input name="target_feature">class</oml:input>
  <oml:input name="estimation_procedure">1</oml:input>
</oml:task_inputs>

> ds = getOMLDataSet(41158)
> ds$target.features
[1] "class"
> task_id = uploadOMLTask("Supervised Classification", ds$desc$id, ds$target.features, estimation.procedure = 1L)
> task_id
[1] 189919

Not working example:

<oml:task_inputs xmlns:oml="http://openml.org/openml">
  <oml:task_type_id>1</oml:task_type_id>
  <oml:input name="source_data">41539</oml:input>
  <oml:input name="target_feature">Rainfall</oml:input>
  <oml:input name="estimation_procedure">1</oml:input>
</oml:task_inputs>

> ds = getOMLDataSet(41539)
> ds$target.features
[1] "Rainfall"
> task_id = uploadOMLTask("Supervised Classification", ds$desc$id, ds$target.features, estimation.procedure = 1L)
 Error in doHTTRCall(method, url = url, query = list(api_key = conf$apikey),  : 
  ERROR (code = 622) in server response: Input value does not match allowed values in foreign column.
  problematic input: target_feature 

Is there something I'm missing? Are there any restrictions on the server side for naming target_feature column?
Would be great, if somebody could help.

Start at the R interface entry point uploadOMLTask and compare the successful dataset 41158 with dataset 41539, focusing on the target_feature value and the server response. Trace where the server rejects the input as an invalid foreign-column value. Done means the cause is handled for the shown reproducer, or the restriction on target-feature names is clearly documented.