Search query crashes php page?

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When I use this search query on my php page running PHRETS :

$results = $rets->Search("Property","RE_1","(L_ListAgent1=|5486, 5943, 7111)",['Limit' => 3]);

the results are as expected. It returns all the residential ('RE_1') listings for those agents.

However when I look at RETS MD and see that there is vacant land I can pull from the database ('VL_5') I try to use the same search query but it crashes my page:

$results = $rets->Search("Property","VL_5","(L_ListAgent1=|5486, 5943, 7111)",['Limit' => 3]);

Why does this happen?