diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,13 @@
 # Changelog
 
+## 4.0.2 (2021-01-23)
+
+#### Fixed
+
+- A bug in zero parsing within SemVer prereleases. [#42]
+
+[#42]: https://github.com/fosskers/versions/issues/42
+
 ## 4.0.1 (2020-10-22)
 
 #### Fixed
diff --git a/Data/Versions.hs b/Data/Versions.hs
--- a/Data/Versions.hs
+++ b/Data/Versions.hs
@@ -788,17 +788,21 @@
 -- since a version like @1.000.1@ would parse as @1.0.1@.
 chunk :: Parsec Void Text VChunk
 chunk = try zeroWithLetters <|> oneZero <|> PC.some (iunit <|> sunit)
-  where oneZero = (:|[]) . Digits . read . T.unpack <$> string "0"
-        zeroWithLetters = do
-          z <- Digits . read . T.unpack <$> string "0"
-          s <- PC.some sunit
-          c <- optional chunk
-          case c of
-            Nothing -> pure $ NEL.cons z s
-            Just c' -> pure $ NEL.cons z s <> c'
+  where
+    oneZero :: Parsec Void Text (NonEmpty VUnit)
+    oneZero = (Digits 0 :| []) <$ single '0'
 
+    zeroWithLetters :: Parsec Void Text (NonEmpty VUnit)
+    zeroWithLetters = do
+      z <- Digits 0 <$ single '0'
+      s <- PC.some sunit
+      c <- optional chunk
+      case c of
+        Nothing -> pure $ NEL.cons z s
+        Just c' -> pure $ NEL.cons z s <> c'
+
 iunit :: Parsec Void Text VUnit
-iunit = Digits . read <$> some digitChar
+iunit = Digits <$> ((0 <$ single '0') <|> (read <$> some digitChar))
 
 sunit :: Parsec Void Text VUnit
 sunit = Str . T.pack <$> some letterChar
diff --git a/test/Test.hs b/test/Test.hs
--- a/test/Test.hs
+++ b/test/Test.hs
@@ -78,7 +78,7 @@
 
 goodSemVs :: [T.Text]
 goodSemVs = [ "0.1.0", "1.2.3", "1.2.3-1", "1.2.3-alpha", "1.2.3-alpha.2"
-            , "1.2.3+a1b2c3.1", "1.2.3-alpha.2+a1b2c3.1"
+            , "1.2.3+a1b2c3.1", "1.2.3-alpha.2+a1b2c3.1", "2.2.1-b05"
             ]
 
 -- | The exact example from `http://semver.org`
@@ -122,6 +122,10 @@
       , testGroup "Whitespace Handling"
         [ testCase "1.2.3-1[ ]" $ parse semver' "semver whitespace" "1.2.3-1 " @?= Right (SemVer 1 2 3 [[Digits 1]] [])
         ]
+      , testGroup "Zero Handling"
+        [ testCase "2.2.1-b05" $ semver "2.2.1-b05" @?= Right (SemVer 2 2 1 [[Str "b", Digits 0, Digits 5]] [])
+
+        ]
       ]
     , testGroup "(Haskell) PVP"
       [ testGroup "Good PVPs" $
@@ -260,7 +264,7 @@
 comp f a b = check $ (<) <$> f a <*> f b
 
 equal :: Ord r => (T.Text -> Either l r) -> T.Text -> Assertion
-equal f a = check $ (\r -> compare r r == EQ) <$> f a
+equal f a = check $ (\r -> r == r) <$> f a
 
 check :: Either a Bool -> Assertion
 check = assertBool "Some Either-based assertion failed" . either (const False) id
diff --git a/versions.cabal b/versions.cabal
--- a/versions.cabal
+++ b/versions.cabal
@@ -1,6 +1,6 @@
 cabal-version:      2.2
 name:               versions
-version:            4.0.1
+version:            4.0.2
 synopsis:           Types and parsers for software version numbers.
 description:
   A library for parsing and comparing software version numbers. We like to give