diff --git a/Example/FileSystemProvider/Main.hs b/Example/FileSystemProvider/Main.hs
--- a/Example/FileSystemProvider/Main.hs
+++ b/Example/FileSystemProvider/Main.hs
@@ -1,5 +1,6 @@
 {-# LANGUAGE AllowAmbiguousTypes #-}
 {-# LANGUAGE TemplateHaskell #-}
+{-# OPTIONS_GHC -fplugin GHC.TypeLits.KnownNat.Solver #-}
 
 -- SPDX-License-Identifier: MPL-2.0
 
diff --git a/Example/Logging/Main.hs b/Example/Logging/Main.hs
--- a/Example/Logging/Main.hs
+++ b/Example/Logging/Main.hs
@@ -1,6 +1,7 @@
 {-# LANGUAGE AllowAmbiguousTypes #-}
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE TemplateHaskell #-}
+{-# OPTIONS_GHC -fplugin GHC.TypeLits.KnownNat.Solver #-}
 
 -- This Source Code Form is subject to the terms of the Mozilla Public
 -- License, v. 2.0. If a copy of the MPL was not distributed with this
diff --git a/README.md b/README.md
--- a/README.md
+++ b/README.md
@@ -73,10 +73,18 @@
         TemplateHaskell,
         PartialTypeSignatures,
         AllowAmbiguousTypes
-
-    ghc-options: ... -fplugin GHC.TypeLits.KnownNat.Solver
 ...
 ```
+
+If you encounter an error like the following, add the pragma:
+
+```haskell
+{-# OPTIONS_GHC -fplugin GHC.TypeLits.KnownNat.Solver #-}
+```
+
+to the header of your source file.
+
+    Could not deduce ‘GHC.TypeNats.KnownNat (1 GHC.TypeNats.+ ...)’
 
 The supported versions are GHC 9.4.1 and later.
 This library has been tested with GHC 9.8.2 and 9.4.1.
diff --git a/heftia-effects.cabal b/heftia-effects.cabal
--- a/heftia-effects.cabal
+++ b/heftia-effects.cabal
@@ -1,6 +1,6 @@
 cabal-version:      2.4
 name:               heftia-effects
-version:            0.4.0.0
+version:            0.4.0.1
 
 -- A short (one-line) description of the package.
 synopsis: higher-order effects done right
@@ -35,7 +35,7 @@
 source-repository head
     type: git
     location: https://github.com/sayo-hs/heftia
-    tag: v0.4.0
+    tag: v0.4.0.1
     subdir: heftia-effects
 
 common common-base
@@ -63,7 +63,7 @@
         ghc-typelits-knownnat ^>= 0.7,
         containers > 0.6.5 && < 0.8,
 
-    ghc-options: -Wall -fplugin GHC.TypeLits.KnownNat.Solver
+    ghc-options: -Wall
 
 library
     import: common-base
diff --git a/src/Control/Monad/Hefty/Concurrent/Timer.hs b/src/Control/Monad/Hefty/Concurrent/Timer.hs
--- a/src/Control/Monad/Hefty/Concurrent/Timer.hs
+++ b/src/Control/Monad/Hefty/Concurrent/Timer.hs
@@ -1,10 +1,10 @@
+{-# OPTIONS_GHC -fplugin GHC.TypeLits.KnownNat.Solver #-}
+
 -- SPDX-License-Identifier: MPL-2.0
 
 module Control.Monad.Hefty.Concurrent.Timer where
 
 import Control.Concurrent.Thread.Delay qualified as Thread
-import Control.Monad.Hefty.Coroutine (runCoroutine)
-import Control.Monad.Hefty.State (evalState)
 import Control.Monad.Hefty (
     interpose,
     interpret,
@@ -17,6 +17,8 @@
     type (<|),
     type (~>),
  )
+import Control.Monad.Hefty.Coroutine (runCoroutine)
+import Control.Monad.Hefty.State (evalState)
 import Data.Effect.Concurrent.Timer (CyclicTimer (Wait), Timer (..), clock, cyclicTimer)
 import Data.Effect.Coroutine (Status (Continue, Done))
 import Data.Effect.State (get, put)