Geting php error for array_merge and array_unique

Below are the arrays and the code I am trying to unique/merge

$data['default_new'] = array_unique(array_merge($data['default'], $data['related']))



Array
(
    [0] => Array
        (
            [keywords_id] => 8
            [keyword] => Curling
            [parent_id] => 5
            [count] => 0
        )

)
Array
(
    [0] => Array
        (
            [keywords_id] => 8
            [keyword] => Curling
            [parent_id] => 5
            [count] => 0
        )

    [1] => Array
        (
            [keywords_id] => 10
            [keyword] => Catchers
            [parent_id] => 6
            [count] => 0
        )

    [2] => Array
        (
            [keywords_id] => 16
            [keyword] => CES 2013
            [parent_id] => 3
            [count] => 0
        )

)

It gives me an array to string error:

A PHP Error was encountered

Severity: Notice

Message: Array to string conversion

Filename: models/content_model.php

Line Number: 29

I had this problem with unique and merge before and never fixed it!

I am using codeigniter

here is more info regarding the function I am using array_unique/merge in:

    public function results($data, $searched)
    {
        $page['searched'] = $searched;
        $page['is_active'] = $this->logic_model->is_active();
        $data2 = array();
        $data2['default_new'] = array_unique(array_merge($data['default'], 
$data['related']));
}

line 29 is the $data2['default_new'] = array_u

the $data parameter contains, default and regular which can be seen above.

vardump of data:

array(3) {
  ["active"]=>
  array(2) {
    ["id"]=>
    string(1) "5"
    ["keyword"]=>
    string(6) "Sports"
  }
  ["related"]=>
  array(1) {
    [0]=>
    array(4) {
      ["keywords_id"]=>
      string(1) "8"
      ["keyword"]=>
      string(7) "Curling"
      ["parent_id"]=>
      string(1) "5"
      ["count"]=>
      string(1) "0"
    }
  }
  ["default"]=>
  array(3) {
    [0]=>
    array(4) {
      ["keywords_id"]=>
      string(1) "8"
      ["keyword"]=>
      string(7) "Curling"
      ["parent_id"]=>
      string(1) "5"
      ["count"]=>
      string(1) "0"
    }
    [1]=>
    array(4) {
      ["keywords_id"]=>
      string(2) "10"
      ["keyword"]=>
      string(8) "Catchers"
      ["parent_id"]=>
      string(1) "6"
      ["count"]=>
      string(1) "0"
    }
    [2]=>
    array(4) {
      ["keywords_id"]=>
      string(2) "16"
      ["keyword"]=>
      string(8) "CES 2013"
      ["parent_id"]=>
      string(1) "3"
      ["count"]=>
      string(1) "0"
    }
  }
}