diff --git a/compiler/rustc_span/src/hygiene.rs b/compiler/rustc_span/src/hygiene.rs
index 9f2ff437842..6f0e544a597 100644
--- a/compiler/rustc_span/src/hygiene.rs
+++ b/compiler/rustc_span/src/hygiene.rs
@@ -1277,11 +1277,11 @@ pub fn register_expn_id(
     let expn_id = ExpnId { krate, local_id };
     HygieneData::with(|hygiene_data| {
         let _old_data = hygiene_data.foreign_expn_data.insert(expn_id, data);
-        debug_assert!(_old_data.is_none());
+        debug_assert!(_old_data.is_none() || cfg!(parallel_compiler));
         let _old_hash = hygiene_data.foreign_expn_hashes.insert(expn_id, hash);
-        debug_assert!(_old_hash.is_none());
+        debug_assert!(_old_hash.is_none() || cfg!(parallel_compiler));
         let _old_id = hygiene_data.expn_hash_to_expn_id.insert(hash, expn_id);
-        debug_assert!(_old_id.is_none());
+        debug_assert!(_old_id.is_none() || cfg!(parallel_compiler));
     });
     expn_id
 }