Not found or invalid function error

Hello,

I just checked my error_log file and found this warning over and over again.. I was hoping someone could explain it and possibility present a fix.

Thank you!

PHP Warning: call_user_func_array() expects parameter 1 to be a valid callback, function ‘improved_trim_excerpt’ not found or invalid function name in /home/thecircl/public_html/wp-includes/plugin.php on line 214

function apply_filters( $tag, $value ) {
global $wp_filter, $merged_filters, $wp_current_filter;

$args = array();

// Do ‘all’ actions first.
if ( isset($wp_filter[‘all’]) ) {
$wp_current_filter[] = $tag;
$args = func_get_args();
_wp_call_all_hook($args);
}

if ( !isset($wp_filter[$tag]) ) {
if ( isset($wp_filter[‘all’]) )
array_pop($wp_current_filter);
return $value;
}

if ( !isset($wp_filter[‘all’]) )
$wp_current_filter[] = $tag;

// Sort.
if ( !isset( $merged_filters[ $tag ] ) ) {
ksort($wp_filter[$tag]);
$merged_filters[ $tag ] = true;
}

reset( $wp_filter[ $tag ] );

if ( empty($args) )
$args = func_get_args();

do {
foreach( (array) current($wp_filter[$tag]) as $the_ )
if ( !is_null($the_[‘function’]) ){
$args[1] = $value;
**LINE 214** $value = call_user_func_array($the_[‘function’], array_slice($args, 1, (int) $the_[‘accepted_args’]));
}
} while ( next($wp_filter[$tag]) !== false );
array_pop( $wp_current_filter );
return $value;
}