diff --git a/.gitignore b/.gitignore
index 59fe709..abda8df 100644
--- a/.gitignore
+++ b/.gitignore
@@ -68,3 +68,4 @@ test/web-files-out.txt
 # Python bytecode from the tools directory
 __pycache__/
 *.pyc
+/forms.so
diff --git a/forms.so b/forms.so
deleted file mode 100755
index bf4727d..0000000
Binary files a/forms.so and /dev/null differ
diff --git a/lib/check.ml b/lib/check.ml
index 9a39427..a593571 100644
--- a/lib/check.ml
+++ b/lib/check.ml
@@ -2073,6 +2073,21 @@ and file_guard ctx loc ~path_slot ~op mk_steps =
     (Tast.Let ([ (ok, mk loc Types.Bool (Tast.Bool false)) ],
                [ mk loc Types.Unit (Tast.While (notok (), [ body ])) ]))
 
+(* Is this bare symbol the name of a type? Every table [resolve_name] will look
+   in, and the union table is one of them: a union is [Named] exactly as a
+   struct is, so (vec-new Form) is as ordinary as (vec-new Cell). It was left
+   out when unions landed, which made the prelude's own (vec-new Form) fail
+   with "nothing here says what (vec-new) is a Vec of" — a message about a
+   missing annotation for a program that had written one. One list, read by
+   both callers, so the next kind of type added cannot be added to one of
+   them. *)
+and type_named ctx n =
+  List.mem n Types.primitive_names
+  || Hashtbl.mem ctx.env.structs n
+  || Hashtbl.mem ctx.env.unions n
+  || Hashtbl.mem ctx.env.enums n
+  || Hashtbl.mem ctx.env.aliases n
+
 (* The element type for [vec-new]: a leading bare symbol naming a type, or the
    expectation at the site. A bare symbol shadowed by a local or a global is
    that binding — an allocator, in practice — and not a type. *)
@@ -2082,10 +2097,7 @@ and vec_new_elem ctx ~want loc args =
     | { Ast.e = Ast.Var n; _ } :: rest
       when lookup ctx n = None
         && (not (Hashtbl.mem ctx.env.globals n))
-        && (List.mem n Types.primitive_names
-            || Hashtbl.mem ctx.env.structs n
-            || Hashtbl.mem ctx.env.enums n
-            || Hashtbl.mem ctx.env.aliases n) ->
+        && type_named ctx n ->
       Some (resolve_name ctx.env ~seen:[] loc n, rest)
     | _ -> None
   in
@@ -2114,10 +2126,7 @@ and map_new_types ctx ~want loc args =
   let is_type n =
     lookup ctx n = None
     && (not (Hashtbl.mem ctx.env.globals n))
-    && (List.mem n Types.primitive_names
-        || Hashtbl.mem ctx.env.structs n
-        || Hashtbl.mem ctx.env.enums n
-        || Hashtbl.mem ctx.env.aliases n)
+    && type_named ctx n
   in
   match args with
   | { Ast.e = Ast.Var k; _ } :: { Ast.e = Ast.Var v; _ } :: rest
diff --git a/test/programs/unions.flan b/test/programs/unions.flan
index 56ad9fb..070687c 100644
--- a/test/programs/unions.flan
+++ b/test/programs/unions.flan
@@ -93,4 +93,18 @@
   (print (Shape.Dot {.x 1.5 .y -2.5})) (println "")
   (print (Shape.Tag {.name "printed" .n 9})) (println "")
   (print (Cell {.id 7 .s (Shape.Rect {.w 1 .h 2})})) (println "")
+
+  ;; A union names an element type the same way a struct does. It reads as
+  ;; trivia and it was not: the type-name test (vec-new) and (map-new) use to
+  ;; read a leading bare symbol listed structs, enums, aliases and primitives
+  ;; and not unions, so (vec-new Shape) was refused for not saying what it
+  ;; held -- by a program that had said.
+  (let [vs (vec-new Shape)
+        ms (map-new string Shape)]
+    (push vs (Shape.Rect {.w 2 .h 3}))
+    (push vs Shape.Empty)
+    (put ms "only" (Shape.Tag {.name "in a map" .n 1}))
+    (print (i64 (area (at vs 0)))) (println "")
+    (println (describe (at vs 1)))
+    (println (match (get ms "only") (Some s) (describe s) None "missing")))
   0)
diff --git a/test/test_acceptance.ml b/test/test_acceptance.ml
index d60033c..81cb83a 100644
--- a/test/test_acceptance.ml
+++ b/test/test_acceptance.ml
@@ -1696,7 +1696,8 @@ ERR@7 unexpected token: not the kind the caller was reading
        32\n0\n-1\nin a cell\nempty\n30\nreassigned\n15\n\
        Shape.Empty\n(Shape.Dot {.x 1.5 .y -2.5})\n\
        (Shape.Tag {.name \"printed\" .n 9})\n\
-       (Cell {.id 7 .s (Shape.Rect {.w 1 .h 2})})\n"
+       (Cell {.id 7 .s (Shape.Rect {.w 1 .h 2})})\n\
+       6\nempty\nin a map\n"
     in
     outputs "unions" "programs/unions.flan" unions_out;
     outputs ~opt:"-O0" "unions, -O0" "programs/unions.flan" unions_out;