diff --git a/Export.lean b/Export.lean
index 8a00d1c..5bef4ab 100644
--- a/Export.lean
+++ b/Export.lean
@@ -19,9 +19,9 @@ structure State where
   noMDataExprs : HashMap Expr Expr := {}
   exportUnsafe : Bool := false
 
-abbrev M := ReaderT Context <| StateT State IO
+abbrev M := ExceptT String <| ReaderT Context <| StateT State IO
 
-def M.run (env : Environment) (act : M α) : IO α :=
+def M.run (env : Environment) (act : M α) : IO (Except String α) :=
   StateT.run' (s := {}) do
     ReaderT.run (r := { env }) do
       act
@@ -46,7 +46,8 @@ def dumpName (n : Name) : M Nat := getIdx n (·.visitedNames) ({ · with visited
 
 def dumpLevel (l : Level) : M Nat := getIdx l (·.visitedLevels) ({ · with visitedLevels := · }) do
   match l with
-  | .zero | .mvar _ => unreachable!
+  | .zero => unreachable!
+  | .mvar _ => throw s!"level mvars may not be exported"
   | .succ l => return s!"#US {← dumpLevel l}"
   | .max l1 l2 => return s!"#UM {← dumpLevel l1} {← dumpLevel l2}"
   | .imax l1 l2 => return s!"#UIM {← dumpLevel l1} {← dumpLevel l2}"
@@ -71,7 +72,8 @@ def removeMData (e : Expr) : M Expr := do
     return x
   let e' ← match e with
   | .mdata _ e' => removeMData e'
-  | .fvar .. | .mvar ..=> unreachable!
+  | .fvar .. => throw "fvar found in removeMData; fvars may not be exported"
+  | .mvar ..=> throw "mvar found in removeMData; mvars may not be exported"
   | .bvar .. | .sort .. | .const .. | .lit _ => pure e
   | .app f a =>
     pure <| e.updateApp! (← removeMData f) (← removeMData a)
@@ -97,7 +99,9 @@ partial def dumpExprAux (e : Expr) : M Nat := do
     | .lam n d b bi => return s!"#EL {dumpInfo bi} {← dumpName n} {← dumpExprAux d} {← dumpExprAux b}"
     | .letE n d v b _ => return s!"#EZ {← dumpName n} {← dumpExprAux d} {← dumpExprAux v} {← dumpExprAux b}"
     | .forallE n d b bi => return s!"#EP {dumpInfo bi} {← dumpName n} {← dumpExprAux d} {← dumpExprAux b}"
-    | .mdata .. | .fvar .. | .mvar .. => unreachable!
+    | .mdata .. => throw "mdata found in dumpExprAux; mdata may not be exported"
+    | .fvar .. => throw "fvar found in dumpExprAux; fvars may not be exported"
+    | .mvar .. => throw "mvar found in dumpExprAux; mvars may not be exported"
     -- extensions compared to Lean 3
     | .proj s i e => return s!"#EJ {← dumpName s} {i} {← dumpExprAux e}"
     | .lit (.natVal i) => return s!"#ELN {i}"
@@ -117,7 +121,10 @@ def dumpUparams (uparams : List Name) : M (List Nat) := do
   pure nameIdxs
 
 partial def dumpConstant (c : Name) : M Unit := do
-  let declar := ((← read).env.find? c).get!
+  let declar ←
+    match ((← read).env.find? c) with
+    | none => throw s!"unable to find constant in current environment `{c}`"
+    | some d => pure d
   if (declar.isUnsafe && !(← get).exportUnsafe) || (← get).visitedConstants.contains c then
     return
   modify fun st => { st with visitedConstants := st.visitedConstants.insert c }
diff --git a/Main.lean b/Main.lean
index 60d597d..7a9dea5 100644
--- a/Main.lean
+++ b/Main.lean
@@ -1,9 +1,9 @@
 import Export
 open Lean
 
-def semver := "2.0.0"
+def semver := "2.1.0"
 
-def main (args : List String) : IO Unit := do
+def main (args : List String) : IO UInt32 := do
   initSearchPath (← findSysroot)
   let (opts, args) := args.partition (fun s => s.startsWith "--" && s.length ≥ 3)
   let (imports, constants) := args.span (· != "--")
@@ -12,9 +12,14 @@ def main (args : List String) : IO Unit := do
   let constants := match constants.tail? with
     | some cs => cs.map fun c => Syntax.decodeNameLit ("`" ++ c) |>.get!
     | none    => env.constants.toList.map Prod.fst |>.filter (!·.isInternal)
-  M.run env do
+  let runResult ← M.run env do
     modify (fun st => { st with exportUnsafe := opts.any (· == "--export-unsafe") })
     IO.println semver
     for c in constants do
       modify (fun st => { st with noMDataExprs := {} })
       let _ ← dumpConstant c
+  match runResult with
+  | .ok _ => return 0
+  | e@(.error _) =>
+    IO.eprintln s!"{e}"
+    return 1
diff --git a/lean-toolchain b/lean-toolchain
index 9407130..7035713 100644
--- a/lean-toolchain
+++ b/lean-toolchain
@@ -1 +1 @@
-leanprover/lean4:v4.25.0-rc2
+leanprover/lean4:v4.26.0-rc2
\ No newline at end of file