diff --git a/vyper/venom/analysis/liveness.py b/vyper/venom/analysis/liveness.py
index ac06ff4dae..5d1ac488f1 100644
--- a/vyper/venom/analysis/liveness.py
+++ b/vyper/venom/analysis/liveness.py
@@ -54,7 +54,8 @@ def _calculate_out_vars(self, bb: IRBasicBlock) -> bool:
         Compute out_vars of basic block.
         Returns True if out_vars changed
         """
-        out_vars = bb.out_vars.copy()
+        out_vars = bb.out_vars
+        bb.out_vars = OrderedSet()
         for out_bb in bb.cfg_out:
             target_vars = self.input_vars_from(bb, out_bb)
             bb.out_vars = bb.out_vars.union(target_vars)