Warning error for call_user_func() in shortcodes.php

For a reason that I dont know of, I have started to get the following error on my website.

Warning: call_user_func() [function.call-user-func]: First argument is expected to be a valid callback in /home/bwhoward/public_html/wp-includes/shortcodes.php on line 236

Line 236 in shortfodes.php is under the function do_shortcode_tag and relates spicicly to the following line of code:

return $m[1] . call_user_func( $shortcode_tags[$tag], $attr, $m[5], $tag ) . $m[6];

I could not find the problem in that or any second immediately before or after it (which I will post at the end). Anyone know how to fix this issue? Thanks ??

function do_shortcode_tag( $m ) {
	global $shortcode_tags;

	// allow foo syntax for escaping a tag
	if ( $m[1] == '[' && $m[6] == ']' ) {
		return substr($m[0], 1, -1);
	}

	$tag = $m[2];
	$attr = shortcode_parse_atts( $m[3] );

	if ( isset( $m[5] ) ) {
		// enclosing tag - extra parameter
		return $m[1] . call_user_func( $shortcode_tags[$tag], $attr, $m[5], $tag ) . $m[6];
	} else {
		// self-closing tag
		return $m[1] . call_user_func( $shortcode_tags[$tag], $attr, null,  $tag ) . $m[6];
	}
}