diff --git a/hugr-core/src/std_extensions/collections.rs b/hugr-core/src/std_extensions/collections.rs
index 5900df4ef..270f234b6 100644
--- a/hugr-core/src/std_extensions/collections.rs
+++ b/hugr-core/src/std_extensions/collections.rs
@@ -164,7 +164,7 @@ impl ListOp {
             insert => self
                 .list_polytype(
                     vec![l.clone(), USIZE_T, e.clone()],
-                    vec![l, either_type(Type::UNIT, e).into()],
+                    vec![l, either_type(e, Type::UNIT).into()],
                 )
                 .into(),
             length => self.list_polytype(vec![l], vec![USIZE_T]).into(),