diff --git a/mypy/nativeparse.py b/mypy/nativeparse.py
index 7b4570856e0f..13676c9058b5 100644
--- a/mypy/nativeparse.py
+++ b/mypy/nativeparse.py
@@ -964,8 +964,7 @@ def read_type(state: State, data: ReadBuffer) -> Type:
         union = UnionType(items, uses_pep604_syntax=uses_pep604_syntax)
         union.original_str_expr = original_str_expr
         union.original_str_fallback = original_str_fallback
-        if original_str_expr is not None:
-            union.is_evaluated = False
+        union.is_evaluated = read_bool(data)
         read_loc(data, union)
         expect_end_tag(data)
         return union