wp_set_object_terms setting only first id from array

Why would wp_set_object_terms save only the first value from an array?

I'm using a frontend form to programmatically create a Woocommerce product. The frontend form uses a multiple select field that dynamically pulls in all product categories where each option value is the ID of each product category term. Here's the form:

<form action="" name="form" method="post" id="edit_image_form" enctype="multipart/form-data">
    <div class="form-group">
        <label>Category</label>
        <select name="image_category[]" id="image_category" class="form-control select-checkbox" multiple />
            <?php 
                $cat_args = array(
                    'orderby'    => 'name',
                    'order'      => 'asc',
                    'hide_empty' => false,
                    'exclude'    => array(72) //Uncategorised
                );
                $product_categories = get_terms( 'product_cat', $cat_args );
                if( !empty($product_categories) ){
                    foreach ($product_categories as $key => $category) {
                        echo '<option value="'.$category->term_id.'">'.$category->name.'</option>';
                    }
                }
            ?>
        </select>
    </div>
    <input type="submit" class="btn btn-primary" value="Add Product" name="submit" />
</form>

After selecting 3 terms from the select field, it spits out the following:

array(3) { 
    [0]=> string(3) "143" 
    [1]=> string(3) "145" 
    [2]=> string(3) "113" 
}

Am I supposed to turn the above array into a comma separated string like below?

$image_category = implode( ", ", $_POST['image_category'] ); //Outputs 143, 145, 113

wp_set_object_terms($product->get_id(), array((int)$image_category), 'product_cat');

If so, it's only saving the first term (143). I can confirm the other three IDs definitely already exist. Where am I going wrong?