Web Services createUser returns 656

I am trying to create a user with the web services method createUser.  It returns with the error number 656.  The description for the error in the Integration Interface Guide states: Object Name is null or empty.

The following is how the method is called:

createUser("abc","xyz","",params);

where params are:

<_user>

  <uuu_user_loginname>dldtest2</uuu_user_loginname>

  <uuu_user_password>A1Pass!</uuu_user_password>

  <uuu_user_status>1</uuu_user_status>

  <uuu_user_firstname>dldtest2</uuu_user_firstname>

  <uuu_user_lastname>dldtest2</uuu_user_lastname>

  <uuu_user_title>dldtest2</uuu_user_title>

  <uuu_user_email>a@b.com