drupal 7 file_save_upload returning false

drupal 7 file_save_upload returning false

I have a custom form, with a field for the user to upload an image file
(their logo). In the form validate hook, I've implemented
file_save_upload, which is continually returning false. I can see that the
file is in fact being saved in the correct location on upload, so why
isn't file_save_upload working?
The form field:
$form['company_logo'] = array(
'#type' => 'managed_file',
'#title' => t('Company Logo'),
'#description' => t('Allowed extensions: gif png jpg jpeg'),
'#upload_location' => 'public://uploads/',
'#upload_validators' => array(
'file_validate_extensions' => array('gif png jpg jpeg'),
// Pass the maximum file size in bytes
//'file_validate_size' => array(MAX_FILE_SIZE*1024*1024),
),
);
The validation hook:
$file = file_save_upload( 'company_logo' , array(),
'public://uploads/', FILE_EXISTS_RENAME);
if (!$file) {
form_set_error('company_logo', t('Unable to access file or file is
missing.'));
}