Changing excerpt length in child theme

Hi everyone – I’ve been researching this and testing and the like, and I can’t seem to get this to work. Here is my code:

<?php

/* Set up actions */
add_action( 'after_setup_theme', 'twentyeleven-child_setup' );

if ( ! function_exists( 'isr_setup' ) ):

function isr_setup() {

    // OVERRIDE : EXCERPT LENGTH FUNCTION
    function isr_clientname_excerpt_length( $length ) {
        return 20;
    }
    remove_filter( 'excerpt_length', 'twentyeleven_excerpt_length' );
    add_filter( 'excerpt_length', 'isr_clientname_excerpt_length' );

}
endif; // isr_setup

?>

My child theme is named twentyeleven-child.

I’m getting a server error with the blank screen showing.

I’m sure this is user error – I just can’t figure it out.

Thanks in advance!