SQL error when using Signup: Enabled/Disabled (Enabled)

Hello,

In views, when creating an view to display content types that have signups enabled, I get the following SQL error: SQLSTATE[42S22]: Column not found: 1054 Unknown column 'signup.nidISNOTNULL' in 'where clause'

the SQL: SELECT node.title AS node_title, node.nid AS nid, signup.close_signup_limit AS signup_close_signup_limit, signup.status AS signup_status, node.created AS node_created
FROM
{node} node
LEFT JOIN {signup} signup ON node.nid = signup.nid
WHERE (( (node.status = '1') AND (node.type IN ('event', 'events_door_leden')) AND (signup.nidISNOTNULL IS NULL ) ))
ORDER BY node_created DESC

Any idea where to start looking?

thx