diff --git a/Pantograph/Expr.lean b/Pantograph/Expr.lean
index 14d063e..7e691d5 100644
--- a/Pantograph/Expr.lean
+++ b/Pantograph/Expr.lean
@@ -30,7 +30,7 @@ def instantiateAll (e: Expr): MetaM Expr := do
 
 structure DelayedMVarInvocation where
   mvarIdPending: MVarId
-  args: Array (FVarId × Expr)
+  args: Array (FVarId × (Option Expr))
   tail: Array Expr
 
 @[export pantograph_to_delayed_mvar_invocation_meta_m]
@@ -38,14 +38,22 @@ def toDelayedMVarInvocation (e: Expr): MetaM (Option DelayedMVarInvocation) := d
   let .mvar mvarId := e.getAppFn | return .none
   let .some decl ← getDelayedMVarAssignment? mvarId | return .none
   let mvarIdPending := decl.mvarIdPending
+  let mvarDecl ← mvarIdPending.getDecl
   -- Print the function application e. See Lean's `withOverApp`
   let args := e.getAppArgs
 
   assert! args.size >= decl.fvars.size
+  let fvarArgMap: HashMap FVarId Expr := HashMap.ofList $ (decl.fvars.map (·.fvarId!) |>.zip args).toList
+  let subst ← mvarDecl.lctx.foldlM (init := []) λ acc localDecl => do
+    let fvarId := localDecl.fvarId
+    let a := fvarArgMap.find? fvarId
+    return acc ++ [(fvarId, a)]
+
+  assert! decl.fvars.all (λ fvar => mvarDecl.lctx.findFVar? fvar |>.isSome)
 
   return .some {
     mvarIdPending,
-    args := decl.fvars.map (·.fvarId!) |>.zip args,
+    args := subst.toArray,
     tail := args.toList.drop decl.fvars.size |>.toArray,
   }
 
diff --git a/Pantograph/Serial.lean b/Pantograph/Serial.lean
index 58d4ea7..3d46ee2 100644
--- a/Pantograph/Serial.lean
+++ b/Pantograph/Serial.lean
@@ -92,12 +92,14 @@ partial def serializeExpressionSexp (expr: Expr) (sanitize: Bool := true): MetaM
   delayedMVarToSexp (e: Expr): MetaM (Option String) := do
     let .some invocation ← toDelayedMVarInvocation e | return .none
     let callee ← self $ ← instantiateMVars $ .mvar invocation.mvarIdPending
-    let sites ← invocation.args.mapM (λ (fvar, arg) => do
-        pure s!"({toString fvar.name} {← self arg})"
+    let sites ← invocation.args.mapM (λ (fvarId, arg) => do
+        let arg := match arg with
+          | .some arg => arg
+          | .none => .fvar fvarId
+        self arg
       )
     let tailArgs ← invocation.tail.mapM self
 
-
     let sites := " ".intercalate sites.toList
     let result := if tailArgs.isEmpty then
         s!"(:subst {callee} {sites})"
diff --git a/Test/Proofs.lean b/Test/Proofs.lean
index 54b5016..b09e403 100644
--- a/Test/Proofs.lean
+++ b/Test/Proofs.lean
@@ -243,10 +243,10 @@ def test_or_comm: TestM Unit := do
   addTest $ LSpec.check "(2 root)" state2.rootExpr?.isNone
 
   let state2parent ← serializeExpressionSexp state2.parentExpr?.get! (sanitize := false)
-  let substHead := "((:c Or.casesOn) (:fv _uniq.10) (:fv _uniq.13) (:lambda t._@._hyg.26 ((:c Or) (:fv _uniq.10) (:fv _uniq.13)) (:forall h ((:c Eq) ((:c Or) (:fv _uniq.10) (:fv _uniq.13)) (:fv _uniq.16) 0) ((:c Or) (:fv _uniq.13) (:fv _uniq.10)))) (:fv _uniq.16) (:lambda h._@._hyg.27 (:fv _uniq.10) (:subst (:lambda h._@._hyg.28 ((:c Eq) ((:c Or) (:fv _uniq.10) (:fv _uniq.13)) (:fv _uniq.16) ((:c Or.inl) (:fv _uniq.10) (:fv _uniq.13) (:fv _uniq.47))) (:subst (:subst (:mv _uniq.59) (_uniq.54 (:fv _uniq.16)) (_uniq.55 (:fv _uniq.50))) (_uniq.50 0))) (_uniq.47 0))) (:lambda h._@._hyg.29 (:fv _uniq.13) (:subst (:lambda h._@._hyg.30 ((:c Eq) ((:c Or) (:fv _uniq.10) (:fv _uniq.13)) (:fv _uniq.16) ((:c Or.inr) (:fv _uniq.10) (:fv _uniq.13) (:fv _uniq.60))) (:subst (:subst (:mv _uniq.72) (_uniq.67 (:fv _uniq.16)) (_uniq.68 (:fv _uniq.63))) (_uniq.63 0))) (_uniq.60 0))))"
-  let extra := "((:c Eq.refl) ((:c Or) (:fv _uniq.10) (:fv _uniq.13)) (:fv _uniq.16)))"
+  let substHead := "((:c Or.casesOn) (:fv _uniq.10) (:fv _uniq.13) (:lambda t._@._hyg.26 ((:c Or) (:fv _uniq.10) (:fv _uniq.13)) (:forall h ((:c Eq) ((:c Or) (:fv _uniq.10) (:fv _uniq.13)) (:fv _uniq.16) 0) ((:c Or) (:fv _uniq.13) (:fv _uniq.10)))) (:fv _uniq.16) (:lambda h._@._hyg.27 (:fv _uniq.10) (:subst (:lambda h._@._hyg.28 ((:c Eq) ((:c Or) (:fv _uniq.10) (:fv _uniq.13)) (:fv _uniq.16) ((:c Or.inl) (:fv _uniq.10) (:fv _uniq.13) (:fv _uniq.47))) (:subst (:subst (:mv _uniq.59) (:fv _uniq.10) (:fv _uniq.13) (:fv _uniq.47) (:fv _uniq.16) (:fv _uniq.50)) (:fv _uniq.10) (:fv _uniq.13) (:fv _uniq.16) (:fv _uniq.47) 0)) (:fv _uniq.10) (:fv _uniq.13) (:fv _uniq.16) 0)) (:lambda h._@._hyg.29 (:fv _uniq.13) (:subst (:lambda h._@._hyg.30 ((:c Eq) ((:c Or) (:fv _uniq.10) (:fv _uniq.13)) (:fv _uniq.16) ((:c Or.inr) (:fv _uniq.10) (:fv _uniq.13) (:fv _uniq.60))) (:subst (:subst (:mv _uniq.72) (:fv _uniq.10) (:fv _uniq.13) (:fv _uniq.60) (:fv _uniq.16) (:fv _uniq.63)) (:fv _uniq.10) (:fv _uniq.13) (:fv _uniq.16) (:fv _uniq.60) 0)) (:fv _uniq.10) (:fv _uniq.13) (:fv _uniq.16) 0)))"
+  let extra := "((:c Eq.refl) ((:c Or) (:fv _uniq.10) (:fv _uniq.13)) (:fv _uniq.16))"
   addTest $ LSpec.test "(2 parent)" (state2parent ==
-    s!"((:subst {substHead} (_uniq.41 (:fv _uniq.16))) {extra}")
+    s!"((:subst {substHead} (:fv _uniq.10) (:fv _uniq.13) (:fv _uniq.16) (:fv _uniq.16)) {extra})")
 
   let state3_1 ← match ← state2.tryTactic (goalId := 0) (tactic := "apply Or.inr") with
     | .success state => pure state
@@ -790,7 +790,7 @@ def test_nat_zero_add_alt: TestM Unit := do
         userName? := .some "conduit",
         target := {
           pp? := .some "?m.79 ?m.68 = (n + 0 = n)",
-          sexp? := .some s!"((:c Eq) (:sort 0) (:subst ((:c Eq) (:mv {eqT}) (:mv {eqL}) (:mv {eqR})) (_uniq.77 (:mv {major}))) ((:c Eq) (:c Nat) ({cNatAdd} (:fv {fvN}) {cNat0}) (:fv {fvN})))",
+          sexp? := .some s!"((:c Eq) (:sort 0) (:subst ((:c Eq) (:mv {eqT}) (:mv {eqL}) (:mv {eqR})) (:fv {fvN}) (:mv {major})) ((:c Eq) (:c Nat) ({cNatAdd} (:fv {fvN}) {cNat0}) (:fv {fvN})))",
         },
         vars := #[{
           name := fvN,