Save and Array of values into a Widget setting

Hi, I have the following code inside a custom Widget

function update( $new_instance, $old_instance ) {
		$instance = $old_instance;

		/* Strip tags (if needed) and update the widget settings. */
		$instance['title'] = strip_tags( $new_instance['title'] );
		$instance['posts_id'] = strip_tags( $new_instance['posts_id'] );
		$instance['post_per_query'] = strip_tags( $new_instance['post_per_query'] );
		$instance['thumb_w'] = strip_tags( $new_instance['thumb_w'] );
		$instance['thumb_h'] = strip_tags( $new_instance['thumb_h'] );
		$instance['show_events'] = strip_tags( $new_instance['show_events'] );

		return $instance;
	}

$instance[‘posts_id’] has to contain an array of values from a multivalues <select> input. Using this code nothing saves on this variable.

The input code is:

<p>
		<label for="<?php echo $this->get_field_id( 'posts_id' ); ?>[]">Selecciona servicios relacionados:</label><br  />
		<select name="<?php echo $this->get_field_id( 'posts_id' ); ?>[]" multiple="multiple" size="5" style="width: 96%; height: auto;">
			<?php
			// The code here generates the <options>
			?>
		</select>
		</p>

Any advice please???