error in calendar

Hi.

I get this error when i want to view all the matches.

Warning: Invalid argument supplied for foreach() in /var/www/fcrfan.dk/public_html/wp-content/plugins/sportspress/includes/class-sp-event.php on line 587

Here is the code line mentioned.
foreach ( $duty_appointments as $duty_appointment ) {

what is missing in this line?

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