About: http://webisa.webdatacommons.org/98619320

Property	Value
http://www.w3.org/1999/02/22-rdf-syntax-ns#type	http://www.w3.org/ns/prov#Entity
http://www.w3.org/ns/prov#value	The syntax looks okay (Your union column names are not the same, but as long as the column types are the same, they should take the first query's names/aliases) - so it probably comes down to your PHP code.
http://www.w3.org/ns/prov#wasQuotedFrom	stackoverflow.com