Complex type contract

I am using the upload widget in UI designer to upload a photo and save it to a document that I have previously defined. I've made a complex contract that contains a child "upload" of type File. When I run the workflow instance, I get the following error:
Error:{ "exception": "class org.bonitasoft.engine.bpm.flownode.FlowNodeExecutionException", "message": "USERNAME=walter.bates | org.bonitasoft.engine.core.process.instance.api.exceptions.SActivityStateExecutionException: org.bonitasoft.engine.core.operation.exception.SOperationExecutionException: org.bonitasoft.engine.expression.exception.SExpressionEvaluationException: FLOW_NODE_INSTANCE_ID=353 | Declared return type class org.bonitasoft.engine.bpm.document.DocumentValue is not compatible with evaluated type class org.bonitasoft.engine.bpm.contract.FileInputValue for expression up" }
BUT, when I make a separate contract of simple type File, the error doesn't show. Could you clear this up for me - why does the error show with the complex contract? Thanks in advance!