Can't use function return value in write context

I am using a ternary function just to check if a value is empty or not.But its returning a fatal error

Fatal error: Can't use function return value in write context in /home/sommelie/food.toogoodtogo.hu/wp-content/themes/...../woocommerce/config.woocommerce.php on line 551

 $shipping = !empty(get_post_meta( $restaurant_id, 'delivery_charge', true )) ? get_post_meta( $restaurant_id, 'delivery_charge', true ) : '0.00';

What could be a possible error?is there any error, i don't think so because it works on my server but not this current one.

I am not understanding why it's happening.This is a part of a wordpress theme.

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I had some trouble when trying to use empty with a function call

empty(someFunction());

You should declare a variable to store the post medatata. Or your code will call 2 times the get_post_meta() if the first call return something. It isn't needed :)

 $postMetadata = get_post_meta( $restaurant_id, 'delivery_charge', true );
 $shipping = !empty($postMetadata) ? $postMetadata : '0.00';

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