diff --git a/lib/x86.ml b/lib/x86.ml
index 07eba99..2a0a5a7 100644
--- a/lib/x86.ml
+++ b/lib/x86.ml
@@ -1776,7 +1776,29 @@ and call_native f ~sym ?(chan = false) ~(args : Tast.expr list) ~rty dst =
   call_sym f.b sym;
   if chan then guard f;
   if not (is_void rty) then begin
-    if is_agg rty then unsupported "aggregate return from %s" sym;
+    (* Unreachable, and it is worth saying why rather than leaving it reading
+       like a gap in the backend. Nothing that crosses this boundary returns an
+       aggregate, by two rules that both live in [check.ml]:
+
+       - every aggregate-valued runtime result comes back through an
+         *out-pointer* the checker allocates, so the Flan-level return type is
+         [Unit] or a scalar. [flan_vec_as_slice] is the one that looks like a
+         counter-example and is not: [check.ml] builds it as [rt loc
+         Types.Unit] and [flan_rt.c] writes the two words through [void *out].
+         Every other [rt] builder in the file answers [Unit], an [Int], a
+         [Ptr], an [Alloc] or a [Handle].
+       - [crossable], which admits [String] and [Slice _] only as "a
+         parameter" and refuses an aggregate return from a [declare] outright.
+
+       So this is a guard against those two rules changing, and not a feature
+       waiting to be written. If one ever does change, the work it names is
+       *SysV classification* and not the internal convention in the header: C
+       returns a 16-byte slice in rax:rdx, and there is no classifier in this
+       file. Refusing is the honest answer until there is. *)
+    if is_agg rty then
+      unsupported
+        "%s returns %s by value, which needs SysV return classification this \
+         backend does not have" sym (Types.to_string rty);
     store_loc f ~reg:(if is_float rty then xmm0 else rax) dst rty
   end