Java DMSAPI – how to remove attribute

I want to remove an attribute from a user (SmDMSUser).

Code snippet using the Java DMSAPI -  

Hashtable<String,Object> attributes = user.getAttributes();

//remove attribute from attributes collection

attributes.remove(attribute);

//set the updated attributes
user.setAttributes(attributes);
result = user.modifyObject();

//result –
removeUserAttribute failed..nArg=1,Arg1=32error:851reason:1002

Also tried calling setAttribute() for each individual attribute but got the same result. 

Any recommendations for how to remove an attribute ? sample code would be helpful.  

Thanks,

Mark