diff --git a/artiq/firmware/libproto_artiq/rpc_proto.rs b/artiq/firmware/libproto_artiq/rpc_proto.rs
index dfd1a1dfa..a7248be38 100644
--- a/artiq/firmware/libproto_artiq/rpc_proto.rs
+++ b/artiq/firmware/libproto_artiq/rpc_proto.rs
@@ -260,8 +260,9 @@ mod tag {
                 Tag::ByteArray => 8,
                 Tag::Tuple(it, arity) => {
                     let mut size = 0;
+                    let mut it = it.clone();
                     for _ in 0..arity {
-                        let tag = it.clone().next().expect("truncated tag");
+                        let tag = it.next().expect("truncated tag");
                         size += tag.size();
                     }
                     size