diff --git a/asmcomp/cmm_helpers.ml b/asmcomp/cmm_helpers.ml
index 34f86776eea0..066ece26fa42 100644
--- a/asmcomp/cmm_helpers.ml
+++ b/asmcomp/cmm_helpers.ml
@@ -2809,7 +2809,7 @@ let emit_gc_roots_table ~symbols cont =
 (* Build preallocated blocks (used for Flambda [Initialize_symbol]
    constructs, and Clambda global module) *)
 
-let preallocate_block cont { Clambda.symbol; exported; tag; fields } =
+let preallocate_block cont { Clambda.symbol; exported; tag; fields; block_desc } =
   let space =
     (* These words will be registered as roots and as such must contain
        valid values, in case we are in no-naked-pointers mode.  Likewise
@@ -2827,9 +2827,8 @@ let preallocate_block cont { Clambda.symbol; exported; tag; fields } =
   in
   let global = Cmmgen_state.(if exported then Global else Local) in
   let symb = (symbol, global) in
-  let data =
-    emit_block symb (block_header tag (List.length fields)) space
-  in
+  let header = block_header ~desc:block_desc tag (List.length fields) in
+  let data = emit_block symb header space in
   Cdata data :: cont
 
 let emit_preallocated_blocks preallocated_blocks cont =
diff --git a/middle_end/flambda/simple_value_approx.ml b/middle_end/flambda/simple_value_approx.ml
index 25380deb8446..dfd86043844e 100644
--- a/middle_end/flambda/simple_value_approx.ml
+++ b/middle_end/flambda/simple_value_approx.ml
@@ -679,14 +679,13 @@ let rec meet_descr ~really_import_approx d1 d2 = match d1, d2 with
       equal_boxed_int bi1 i1 bi2 i2 ->
       d1
   | Value_block (tag1, a1, desc1), Value_block (tag2, a2, desc2)
-    when Tag.compare tag1 tag2 = 0 && Array.length a1 = Array.length a2 ->
+    when Tag.compare tag1 tag2 = 0 && Array.length a1 = Array.length a2 &&
+         Block_desc.compare desc1 desc2 = 0
+    ->
     let fields =
       Array.mapi (fun i v -> meet ~really_import_approx v a2.(i)) a1
     in
-    let desc =
-      if Block_desc.compare desc1 desc2 = 0 then desc1 else Block_desc.empty
-    in
-    Value_block (tag1, fields, desc)
+    Value_block (tag1, fields, desc1)
   | _ -> Value_unknown Other
 
 and meet ~really_import_approx a1 a2 =