Invalid argument supplied

When creating an event enabled module (with resource conflict check enabled), I get this PHP error:

"# warning: Invalid argument supplied for foreach() in /var/www/drupaldec/modules/resource_conflict/resource_conflict.module on line 14.
# warning: Invalid argument supplied for foreach() in /var/www/drupaldec/modules/resource_conflict/resource_conflict.module on line 20."

Ignoring my paths, the lines affected are
14: foreach($overlap as $nid) {
15: $overlap_nodes[] = node_load($nid);
16: }

and

20: foreach ($overlap_nodes as $conflict) {

Maybe it's a problem of my configuration, or a general problem, I don't know.

And it could be related to my custom type events are not showing on the events calendar, I'm not sure (I'm debugging it).

Greetings.