diff --git a/Pantograph/Serial.lean b/Pantograph/Serial.lean
index 950818e..26b0c9f 100644
--- a/Pantograph/Serial.lean
+++ b/Pantograph/Serial.lean
@@ -102,9 +102,12 @@ partial def serializeExpressionSexp (expr: Expr) (sanitize: Bool := true): MetaM
     | .fvar fvarId =>
       let name := ofName fvarId.name
       pure s!"(:fv {name})"
-    | .mvar mvarId =>
-      let name := ofName mvarId.name
-      pure s!"(:mv {name})"
+    | .mvar mvarId => do
+      if ← mvarId.isDelayedAssigned then
+        pure s!"(:mv)"
+      else
+        let name := ofName mvarId.name
+        pure s!"(:mv {name})"
     | .sort level =>
       let level := serializeSortLevel level sanitize
       pure s!"(:sort {level})"
diff --git a/Test/Proofs.lean b/Test/Proofs.lean
index 429d4d2..8e77227 100644
--- a/Test/Proofs.lean
+++ b/Test/Proofs.lean
@@ -245,7 +245,7 @@ def test_or_comm: TestM Unit := do
   let state2parent ← serializeExpressionSexp state2.parentExpr?.get! (sanitize := false)
   -- This is due to delayed assignment
   addTest $ LSpec.test "(2 parent)" (state2parent ==
-    "((:mv _uniq.43) (:fv _uniq.16) ((:c Eq.refl) ((:c Or) (:fv _uniq.10) (:fv _uniq.13)) (:fv _uniq.16)))")
+    "((:mv) (:fv _uniq.16) ((:c Eq.refl) ((:c Or) (:fv _uniq.10) (:fv _uniq.13)) (:fv _uniq.16)))")
 
   let state3_1 ← match ← state2.tryTactic (goalId := 0) (tactic := "apply Or.inr") with
     | .success state => pure state