What's the correct format for submitting a null value to an integer field using metadata wrapper? Problem

Whenever I submit a nothing to an integer field using the EntityMetadataWrapper I get the following error. 

What's the correct format for submitting a null value to an integer field? (I don't want 0, but a blank value).


  EntityMetadataWrapperException: Invalid data value given. Be sure it matches the required data type and format. in
  EntityMetadataWrapper->set() (line 122 of
  C:\XAMPP\htdocs\drupal\sites\all\modules\entity\includes\entity.wrapper.inc).


How do you clear a field value with entity_metadata_wrapper()?

The post above implies I should be able to use:

$wrapper-&gt;title = '';


However this still throws the error.

Example

I have an integer field Salary added to a custom node type. (Along with several other fields which work as expected.)

Properties:


Not required 
Has no minimum value
No default value 
Can only hold one value.


I have a custom form which looks like this. This is the entry field:

$form['field_salary'] = array(
    '#type' =&gt; 'textfield',
    '#title' =&gt; t('Salary'),
    '#size' =&gt; 10,
    '#maxlength' =&gt; 10,
    '#description' =&gt; t('Please enter a valid number'),


This is the submit function taken from here:

function postajob_form_submit($form, &amp;$form_state) {

 $values = array(
      'type' =&gt; 'job_advert',
      'uid' =&gt; $user-&gt;uid,
      'status' =&gt; 1,
      'comment' =&gt; 1,
      'promote' =&gt; 0,
    );
    $entity = entity_create('node', $values);

    $ewrapper = entity_metadata_wrapper('node', $entity);

    //.... (other fields in the form are here)
    $ewrapper-&gt;field_salary-&gt;set($form_state['values']['field_salary']);


    $ref_nid = 15;

    $my_date = new DateTime('January 1, 2013');
    $entity-&gt;field_my_date[LANGUAGE_NONE][0] = array(
       'value' =&gt; date_format($my_date, 'Y-m-d'),
       'timezone' =&gt; 'UTC',
       'timezone_db' =&gt; 'UTC',
     );

    $ewrapper-&gt;save(true);
    entity_save('node', $entity);

}


My guess is that it can't hold nothing? But that seems weird because it's not required. 

I can for example create the node using the drupal default form with no errors and it works fine.

EDIT:

The solution from Wayne, removes the error but what I was hoping for was the ability to submit a blank value without it throwing an error. Unfortunately the following doesn't work. 

'#default_value' =&gt; NULL, 


I've considered using a string field instead of an integer one and then validating only for numbers, but this means I'll lose the ability to use the numeric range search facets, again not ideal.
Have you tried to check if the value exists before setting it?

//.... (other fields in the form are here)
if (!empty($form_state['values']['field_salary'])) {
  $ewrapper-&gt;field_salary-&gt;set($form_state['values']['field_salary']);
}


This way you're not touching the value if it's not set.