Parentheses allowed in Custom Form Field Labels

Are parentheses not allowed in Custom Form Field Labels? A user asked me to create a field on a Custom Form with a label that included open/close parentheses. When I type those characters as part of the name (label) for the field, Workfront doesn't recognize that I'd inputted anything; it simply doesn't respond. Is this by design or am I experiencing an issue somehow? If open/close parentheses are disallowed characters, is there a reason why, and are there any other characters I can't use as part of a field label? Thanks in advance. :) M.L. de la Rosa Genentech