diff --git a/gasket_page_table.c b/gasket_page_table.c
index 9a239f0..a29426a 100644
--- a/gasket_page_table.c
+++ b/gasket_page_table.c
@@ -586,7 +586,7 @@ static ulong gasket_extended_lvl0_page_idx(struct gasket_page_table *pg_tbl,
 					   u64 dev_addr)
 {
 	return (dev_addr >> GASKET_EXTENDED_LVL0_SHIFT) &
-	       ((1 << GASKET_EXTENDED_LVL0_WIDTH) - 1);
+	       (pg_tbl->config.total_entries - 1);
 }
 
 /*