diff --git a/src/comp/middle/trans.rs b/src/comp/middle/trans.rs
index 7a55be104fd..78a7a16aca8 100644
--- a/src/comp/middle/trans.rs
+++ b/src/comp/middle/trans.rs
@@ -1231,6 +1231,9 @@ fn simplify_type(&@crate_ctxt ccx, &ty::t typ) -> ty::t {
             case (ty::ty_box(_)) {
                 ret ty::mk_imm_box(ccx.tcx, ty::mk_nil(ccx.tcx));
             }
+            case (ty::ty_vec(_)) {
+                ret ty::mk_imm_vec(ccx.tcx, ty::mk_nil(ccx.tcx));
+            }
             case (_) { ret typ; }
         }
     }
diff --git a/src/comp/middle/ty.rs b/src/comp/middle/ty.rs
index 9f62f2b2ed3..42bd68af6e2 100644
--- a/src/comp/middle/ty.rs
+++ b/src/comp/middle/ty.rs
@@ -427,6 +427,11 @@ fn mk_imm_box(&ctxt cx, &t ty) -> t {
 }
 
 fn mk_vec(&ctxt cx, &mt tm) -> t  { ret gen_ty(cx, ty_vec(tm)); }
+
+fn mk_imm_vec(&ctxt cx, &t typ) -> t {
+    ret gen_ty(cx, ty_vec(rec(ty=typ, mut=ast::imm)));
+}
+
 fn mk_port(&ctxt cx, &t ty) -> t  { ret gen_ty(cx, ty_port(ty)); }
 fn mk_chan(&ctxt cx, &t ty) -> t  { ret gen_ty(cx, ty_chan(ty)); }
 fn mk_task(&ctxt cx) -> t        { ret gen_ty(cx, ty_task); }