Saving Empty Value To Single Value Managed Metadata Field With CSOM

I’ve worked out how to delete the items in a multi-value managed metadata field, detailed in the answer to this question

http://social.msdn.microsoft.com/Forums/sharepoint/en-US/150be0f4-31d1-4c6b-b784-33532c102f08/saving-empty-value-to-managed-metadata-field-with-csom?forum=appsforsharepoint

I’ve tried a similar approach for a single value managed metadata value field, for example

var _field = _caseStudiesList.get_fields().getByInternalNameOrTitle("ProductType");

var _productTypeField = clientContext.castTo(_field, SP.Taxonomy.TaxonomyField);

and then

_productTypeField.setFieldValueByValue(_listItem, null);

Or

var singleManagedMetadatavValue = new SP.Taxonomy.TaxonomyFieldValue();

_productTypeField.setFieldValueByValue(_listItem, _singleManagedMetadatavValue);

Or

var singleManagedMetadatavValue = new SP.Taxonomy.TaxonomyFieldValue();

singleManagedMetadatavValue = null;

_productTypeField.setFieldValueByValue(_listItem, _singleManagedMetadatavValue);

Or

_singleManagedMetadatavValue = new SP.Taxonomy.TaxonomyFieldValue();

_singleManagedMetadatavValue.set_label(null);

_singleManagedMetadatavValue.set_termGuid(null);

_singleManagedMetadatavValue.set_wssId(null);

_productTypeField.setFieldValueByValue(_listItem, _singleManagedMetadatavValue);

I get errors that either state taxValue cannot be null or request failed at TaxonomyFieldValue.get_TermGuid().

Has anyone worked out how to do this?

Thanks

Neil