Identifying Suspect Links in JRS for DNG 6.0.6.1

OPTIONAL{?ns_validity__ValidityLink1_uri <http://jazz.net/ns/validity#status> ?status_enum .}

Should get the link validity status from the variable ?ns_validity__ValidityLink1_uri 
and set the variable ?status_enum with the value.

Then the line:

BIND (IF (!bound(?status_enum), "Suspect", ?status_enum) as ?validity_status)

From the Jazz Linked Data Vocabulary, that value is only Valid or Invalid. If it's Suspect
then ?status_enum should be empty since Suspect is not in the vocabulary. So the 
IF statement checks if ?status_enum is empty and if so sets ?validity_status to "Suspect". 

But, it doesn't work. If the link is Valid or Invalid then the artifact and the link 
status is captured. If it's Suspect, it's not. Any suggestions?