XSpear output is not able to save to a file in json format

XSpear output is not able to save to a file injson format. Tried with below commands. Can you please help to provide the method of saving the output to json file.

XSpear -u 'http://testphp.vulnweb.com/listproducts.php?cat=123' -v 3 -b "https://hahwul.xss.ht" -o filename.json
XSpear -u 'http://testphp.vulnweb.com/listproducts.php?cat=123' -v 3 -b "https://hahwul.xss.ht" --output=filename.json