diff --git a/src/kyupy/sim.py b/src/kyupy/sim.py
index 4729be2..29ac420 100644
--- a/src/kyupy/sim.py
+++ b/src/kyupy/sim.py
@@ -229,7 +229,8 @@ class SimOps:
                     prev_line = prev_line.driver.ins[0]
                 stem_idx = prev_line.index
                 for ol in f.outs:
-                    stems[ol] = stem_idx
+                    if ol is not None:
+                        stems[ol] = stem_idx
 
         # calculate level (distance from PI/PPI) and reference count for each line
         levels = np.zeros(self.c_locs_len, dtype='int32')