in_array(): Argument #2 ($haystack) must be of type array

Why am i having an

Type: TypeError
Message: in_array(): Argument #2 ($haystack) must be of type array, int given

my code is something like this hope someone can help i’m using codeigniter 3

<div class="form-group">
<label for="sub_category">Category:</label>
<?php $sub_category_data = json_decode($product_data['sub_category_id']); ?>
<select class="form-control select_group" id="sub_category_1" name="sub_category[]" onchange="getSubCategoryData(1)">
<?php foreach ($sub_category as $k => $v): ?>
<option value="<?php echo $v['id'] ?>" <?php if(in_array($v['id'], $sub_category_data)) { echo 'selected="selected"'; } ?>><?php echo $v['name'] ?></option>
<?php endforeach ?>
</select>
</div>