custom fields erros when no value available

I’ve set up posts to have several relevant custom fields.
each post may use a few or all of the fields that i’ve set up. however when a post has no need for some of the fields and i have not used them, i can see an error message appear for that particular line on the page.

i have added a conditional statement for each but the error message seems to still prevail.

the code looks something like this

<?php
									if (empty ($custom_fields) )
										{
											echo "";
										}
									else
										{
											$custom_fields = get_post_custom();
											$Track1 = $custom_fields['Track13'];
											foreach ( $Track13 as $key => $value )
											echo  $value . "<br />";
										}
							?>

and the error i get is like this relating to the foreach

Warning: Invalid argument supplied for foreach() in /content/BusinessHostPlus/b/u/burntprogress.com/web/promo/wp-content/themes/burntpromo/single.php on line 260

any idea what i’m doing wrong?