diff --git a/src/kyupy/sim.py b/src/kyupy/sim.py
index d7e443e..0871a42 100644
--- a/src/kyupy/sim.py
+++ b/src/kyupy/sim.py
@@ -191,7 +191,7 @@ class SimOps:
         root_nodes = set([n for n in circuit.s_nodes if len(n.ins) > 0] + [n for n in circuit.nodes if len(n.outs) == 0])  # start from POs, PPOs, and any dangling nodes
         readers = np.array([1 if l.reader in root_nodes else len(l.reader.outs) for l in circuit.lines], dtype=np.int32)  # for ref-counting forks
 
-        level_lines = [n.ins[0] for n in root_nodes]
+        level_lines = [n.ins[0] for n in root_nodes if len(n.ins) > 0 ]
         # FIXME: Should probably instanciate buffers for PPOs and attach DFF clocks
 
         while len(level_lines) > 0:  # traverse the circuit level-wise back towards (P)PIs