diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,9 @@
 # Revision history for timestats
 
+## 0.2.1 -- 2025-07-24
+
+* Avoid negative measures when measuring the lazy State monad (#5).
+
 ## 0.2.0 -- 2024-05-24
 
 * Expose `Debug.TimeStats.measureMWithLiftIO`.
diff --git a/src/Debug/TimeStats.hs b/src/Debug/TimeStats.hs
--- a/src/Debug/TimeStats.hs
+++ b/src/Debug/TimeStats.hs
@@ -1,3 +1,4 @@
+{-# LANGUAGE BangPatterns #-}
 {-# LANGUAGE RankNTypes #-}
 -- | A module to collect aggregates on how much time is spent in a computation
 --
@@ -243,10 +244,10 @@
 -- the given reference to time stats, using the given IO lifting function.
 measureMWithRef :: Monad m => (forall b. IO b -> m b) -> TimeStatsRef -> m a -> m a
 measureMWithRef lift tref m = do
-    t0 <- lift getMonotonicTimeNSec
+    !t0 <- lift getMonotonicTimeNSec
     a <- m
     lift $ do
-      tf <- getMonotonicTimeNSec
+      !tf <- getMonotonicTimeNSec
       updateTimeStatsRef tref $ \st ->
         st
           { timeStat = (tf - t0) + timeStat st
diff --git a/src/Debug/TimeStats/Unsafe.hs b/src/Debug/TimeStats/Unsafe.hs
--- a/src/Debug/TimeStats/Unsafe.hs
+++ b/src/Debug/TimeStats/Unsafe.hs
@@ -9,11 +9,7 @@
   ( unsafeMeasureM
   ) where
 
-import Debug.TimeStats
-         ( lookupTimeStatsRef
-         , measureMWithLiftIO
-         )
-import GHC.Clock (getMonotonicTimeNSec)
+import Debug.TimeStats (measureMWithLiftIO)
 import System.IO.Unsafe (unsafePerformIO)
 
 -- | Like 'Debug.TimeStats.measureM' but can measure other monads.
diff --git a/tests/Main.hs b/tests/Main.hs
--- a/tests/Main.hs
+++ b/tests/Main.hs
@@ -1,10 +1,13 @@
+{-# OPTIONS_GHC -Wno-x-partial #-}
 module Main where
 
 import Control.Exception (evaluate)
 import Control.Monad (unless)
+import Control.Monad.State
 import qualified Data.Text.IO as Text
 import qualified Debug.TimeStats as TimeStats
 import qualified Debug.TimeStats.Internal as Internal
+import qualified Debug.TimeStats.Unsafe as TimeStats
 import System.Environment (setEnv)
 import System.Exit (exitFailure)
 
@@ -14,6 +17,7 @@
 main :: IO ()
 main = do
     testMeasureM
+    testUnsafeMeasureM
     testFormatIntWithSeparator
 
 testMeasureM :: IO ()
@@ -28,13 +32,9 @@
           , ("fib2", TimeStats.TimeStats 0 2)
           ]
     unless (eqStats xs expected) $ do
-      putStrLn "unexpected timestats:"
+      putStrLn "measureM: unexpected timestats:"
       Text.putStrLn (TimeStats.asText xs)
       exitFailure
-  where
-    eqStats xs ys = length xs == length ys && and (zipWith eqStat xs ys)
-    eqStat (lbl0, ts0) (lbl1, ts1) =
-      lbl0 == lbl1 && TimeStats.countStat ts0 == TimeStats.countStat ts1
 
 testFormatIntWithSeparator :: IO ()
 testFormatIntWithSeparator = do
@@ -65,3 +65,27 @@
         putStrLn $ "expected: " ++ show (expected ++ "a")
         putStrLn $ "  actual: " ++ show actual
         exitFailure
+
+testUnsafeMeasureM :: IO ()
+testUnsafeMeasureM = do
+    setEnv "DEBUG_TIMESTATS_ENABLE" "1"
+    TimeStats.reset
+    _ <- evaluate $ (`execState` 0) $
+      TimeStats.unsafeMeasureM "fib" $ put (fib 40)
+    xs <- TimeStats.collect
+    let expected =
+          [ ("fib", TimeStats.TimeStats 0 1)
+          , ("fib2", TimeStats.TimeStats 0 0)
+          ]
+    -- A large time value indicates an error
+    unless (eqStats xs expected
+             || TimeStats.timeStat (snd $ head xs) > 1000000000) $ do
+      putStrLn "unsafeMeasureM: unexpected timestats:"
+      Text.putStrLn (TimeStats.asText xs)
+      exitFailure
+
+eqStats :: Eq a => [(a, TimeStats.TimeStats)] -> [(a, TimeStats.TimeStats)] -> Bool
+eqStats xs ys = length xs == length ys && and (zipWith eqStat xs ys)
+  where
+    eqStat (lbl0, ts0) (lbl1, ts1) =
+      lbl0 == lbl1 && TimeStats.countStat ts0 == TimeStats.countStat ts1
diff --git a/timestats.cabal b/timestats.cabal
--- a/timestats.cabal
+++ b/timestats.cabal
@@ -1,8 +1,12 @@
 cabal-version:      2.4
 name:               timestats
-version:            0.2.0
+version:            0.2.1
 
 synopsis: A library for profiling time in Haskell applications
+description: This library provides some utilities for instrumenting Haskell
+             applications to collect time measures of various fragments of
+             code. This is useful to measure wall-clock time when running a
+             given piece of Haskell code. See the README for more details.
 
 homepage:    https://github.com/tweag/timestats
 bug-reports: https://github.com/tweag/timestats/issues
@@ -13,10 +17,14 @@
 maintainer: facundo.dominguez@tweag.io
 
 category: Profiling
-extra-source-files:
+extra-doc-files:
     CHANGELOG.md
     README.md
 
+source-repository head
+  type:     git
+  location: https://github.com/tweag/timestats
+
 flag devel
   default:     False
   manual:      True
@@ -36,7 +44,7 @@
 test-suite tests
   type: exitcode-stdio-1.0
   main-is: Main.hs
-  build-depends: base, text, timestats
+  build-depends: base, mtl, text, timestats
   if flag(devel)
     ghc-options: -Wall -Werror
   hs-source-dirs: tests