diff --git a/vyper/codegen/core.py b/vyper/codegen/core.py
index 54d773dbd7..5b16938e99 100644
--- a/vyper/codegen/core.py
+++ b/vyper/codegen/core.py
@@ -957,6 +957,7 @@ def _complex_make_setter(left, right):
                 # kind of arbitrary, but cut off when code used > ~160 bytes
                 should_batch_copy = len_ >= 32 * 10
             else:
+                assert _opt_none()
                 # don't care, just generate the most readable version
                 should_batch_copy = True
         else: