diff --git a/src/rstd/collections/pair.rs b/src/rstd/collections/pair.rs
index bc996ba..17ea4fb 100644
--- a/src/rstd/collections/pair.rs
+++ b/src/rstd/collections/pair.rs
@@ -184,6 +184,14 @@ where
     }
 }
 
+impl<'a, Ctx: Context<'a>, FA, FB> ConstSizeFactory<'a, Ctx> for (FA, FB)
+where
+    FA: ConstSizeFactory<'a, Ctx>,
+    FB: ConstSizeFactory<'a, Ctx> + FactoryModeParse<'a, Ctx>,
+{
+    const SIZE: usize = FA::SIZE + FB::SIZE;
+}
+
 impl<A: InliningAtomic, B: AtomicModeParse> AtomicModeParse for (A, B) {
     fn ma_deserialize<I: Stream>(stream: I) -> AModeResultM<Self, I> {
         StaticPairObject::<Self>::ma_deserialize_sp(stream)