diff --git a/charon-ml/src/LlbcAstUtils.ml b/charon-ml/src/LlbcAstUtils.ml
index 4670fdef..5cf18347 100644
--- a/charon-ml/src/LlbcAstUtils.ml
+++ b/charon-ml/src/LlbcAstUtils.ml
@@ -40,7 +40,7 @@ let fun_decl_has_loops (fd : fun_decl) : bool =
 
 (** Create a sequence *)
 let mk_sequence (st1 : statement) (st2 : statement) : statement =
-  let span = MetaUtils.combine_span st1.span st2.span in
+  let span = MetaUtils.safe_combine_span st1.span st2.span in
   let content = Sequence (st1, st2) in
   { span; content; comments_before = [] }
 
@@ -58,9 +58,9 @@ let rec chain_statements (st1 : statement) (st2 : statement) : statement =
       (* Ignore the second statement, which won't be evaluated *) st1
   | Switch switch ->
       (* Insert inside the switch *)
-      let span = MetaUtils.combine_span st1.span st2.span in
+      let span = MetaUtils.safe_combine_span st1.span st2.span in
       let content = Switch (chain_statements_in_switch switch st2) in
-      { span; content; comments_before = [] }
+      { span; content; comments_before = st1.comments_before }
   | Sequence (st3, st4) ->
       (* Insert at the end of the statement *)
       mk_sequence st3 (chain_statements st4 st2)
@@ -79,11 +79,7 @@ and chain_statements_in_switch (switch : switch) (st : statement) : switch =
       let branches =
         List.map (fun (svl, br) -> (svl, chain_statements br st)) branches
       in
-      let otherwise =
-        match otherwise with
-        | None -> None
-        | Some otherwise -> Some (chain_statements otherwise st)
-      in
+      let otherwise = Option.map (fun b -> chain_statements b st) otherwise in
       Match (op, branches, otherwise)
 
 (** Compute a map from function declaration ids to declaration groups. *)
diff --git a/charon-ml/src/MetaUtils.ml b/charon-ml/src/MetaUtils.ml
index d9c1b823..e5c4e29b 100644
--- a/charon-ml/src/MetaUtils.ml
+++ b/charon-ml/src/MetaUtils.ml
@@ -11,13 +11,19 @@ let loc_max (l0 : loc) (l1 : loc) : loc =
   else l1
 
 (** See the comments in [meta_utils.rs] in Charon. *)
-let combine_span (m0 : span) (m1 : span) : span =
-  assert (m0.span.file = m1.span.file);
-  let span =
-    {
-      file = m0.span.file;
-      beg_loc = loc_min m0.span.beg_loc m1.span.beg_loc;
-      end_loc = loc_max m0.span.end_loc m1.span.end_loc;
-    }
-  in
-  { span; generated_from_span = None }
+let combine_span (m0 : span) (m1 : span) : span option =
+  if m0.span.file = m1.span.file then
+    let span =
+      {
+        file = m0.span.file;
+        beg_loc = loc_min m0.span.beg_loc m1.span.beg_loc;
+        end_loc = loc_max m0.span.end_loc m1.span.end_loc;
+      }
+    in
+    Some { span; generated_from_span = None }
+  else None
+
+(** Safely combine two spans, even if they do not come from the same file -
+    if this happens, we simply use the first span. *)
+let safe_combine_span (m0 : span) (m1 : span) : span =
+  match combine_span m0 m1 with None -> m0 | Some m -> m