Cannot use MultiGrader object inside FineTune service API

[lminardi1998]

I am trying to use a MultiGrader object inside the reinforcement method of FineTune API.
For instance, I am validating first the object using GraderService validate method. This is my object creation and request:
MultiGrader testMultiGrader = MultiGrader.builder().name("ASDF").calculateOutput("0.5 * compliant").graders(MultiGrader.Graders.ofStringCheckGrader( StringCheckGrader.builder() .name("compliant") .input("{{sample.output_json.compliant}}") .reference("{{item.compliant}}") .operation(StringCheckGrader.Operation.EQ) .build() )).build(); xxx.fineTuning().alpha().graders().validate( GraderValidateParams.builder() .grader(GraderValidateParams.Grader.ofMulti(testMultiGrader)) .build() );
The result is an error: "Multigrader validation test failed: 400: Invalid type for 'graders.input': expected an object, but got a string instead.". Is there a bug in the API or is me making an error during request creation?