diff --git a/devito/passes/iet/parpragma.py b/devito/passes/iet/parpragma.py
index 9eb6da047f1..9639b27ffa0 100644
--- a/devito/passes/iet/parpragma.py
+++ b/devito/passes/iet/parpragma.py
@@ -614,7 +614,7 @@ def _make_partree(self, candidates, nthreads=None, index=None):
         if self._is_offloadable(root):
             body = self.DeviceIteration(gpu_fit=self.gpu_fit,
                                         ncollapsed=len(collapsable) + 1,
-                                        tile=self.par_tile,
+                                        tile=self.par_tile.next(),
                                         **root.args)
             partree = ParallelTree([], body, nthreads=nthreads)
 
diff --git a/devito/tools/data_structures.py b/devito/tools/data_structures.py
index 13c3e68d9b9..95f9f65d0e9 100644
--- a/devito/tools/data_structures.py
+++ b/devito/tools/data_structures.py
@@ -626,6 +626,8 @@ def __init__(self, items):
         self.current = 0
 
     def next(self):
+        if self.last == 0:
+            return None
         item = self.items[self.current]
         self.current = min(self.last - 1, self.current+1)
         return item