Reports throwing quickform fatal error

Array
(
[callback] => Array
(
[0] => CRM_Core_Error
[1] => handle
)

[code] => -1
[message] => QuickForm Error: unknown error
[mode] => 16
[debug_info] => Form rule callback returned invalid value in HTML_QuickForm::validate()
[type] => HTML_QuickForm_Error
[user_info] => Form rule callback returned invalid value in HTML_QuickForm::validate()
[to_string] => [html_quickform_error: message="unknown error" code=-1 mode=callback callback=CRM_Core_Error::handle prefix="QuickForm Error: " info="Form rule callback returned invalid value in HTML_QuickForm::validate()"]
)

Above error is thrown for following reports when we hit "preview report" button for instance / template -

• contribute repeat report
• activity summary report