** Using WeBWorK **

OK, you left out of your original message the crucial information that you had redefined the open brace using

   Context()->parens->replace('{' => {close => '}', type => 'Set'});

which is the source of the problem. You want

   Context()->parens->replace('{' => {close => '}', type => 'Set', emptyOK=>1});

to allow empty braces to be allowed.

I'm not sure why you are redefining the braces, ass this new definition (with emptyOK should be exactly what is already in the Interval context, so I'd recommend removing the line entirely. If you way what you are trying to accomplish, I may be able to suggest an alternative.