Custom field – some giving errors if not used.

Hello all!

I finally managed to get my custom fields into my ppsts just the way I wanted. HOWEVER, I’ve discovered, if I use ,say, only 2 of the three set out below, the one missing will show up as an error:

Warning: Invalid argument supplied for foreach() in /home/.../...

How can I make the Custom fields work and have the non-showing ones not wreak havoc?

<span class="meta-data">
<?php

  $mykey_values = get_post_custom_values('Client');
  foreach ( $mykey_values as $key => $value ) {
    echo "<strong>Client:</strong> $value<br />";}

?>
<?php

  $mykey_values = get_post_custom_values('Brief');
  foreach ( $mykey_values as $key => $value ) {
    echo "<strong>Brief:</strong> $value<br />";
  }

?>
<?php

  $mykey_values = get_post_custom_values('Media');
  foreach ( $mykey_values as $key => $value ) {
    echo "<strong>Media:</strong> $value<br />";}

?></span>