diff --git a/CHANGES.txt b/CHANGES.txt
--- a/CHANGES.txt
+++ b/CHANGES.txt
@@ -1,5 +1,7 @@
 Changelog for HLint (* = breaking change)
 
+2.1.26, released 2019-06-26
+    Make sure unknown extensions don't cause errors
 2.1.25, released 2019-06-26
     #681, fix for extensions on the command line not being used
     #686, suggest head (drop n x) ==> x !! max 0 n
diff --git a/hlint.cabal b/hlint.cabal
--- a/hlint.cabal
+++ b/hlint.cabal
@@ -1,7 +1,7 @@
 cabal-version:      >= 1.18
 build-type:         Simple
 name:               hlint
-version:            2.1.25
+version:            2.1.26
 license:            BSD3
 license-file:       LICENSE
 category:           Development
diff --git a/src/GHC/Util.hs b/src/GHC/Util.hs
--- a/src/GHC/Util.hs
+++ b/src/GHC/Util.hs
@@ -11,7 +11,7 @@
   , getMessages
   , SDoc
   , Located
-  , hseToGhcExtension
+  , readExtension
   -- Temporary : Export these so GHC doesn't consider them unused and
   -- tell weeder to ignore them.
   , isAtom, addParen, paren, isApp, isOpApp, isAnyApp, isDot, isSection, isDotApp
@@ -39,7 +39,6 @@
 import Data.List
 import System.FilePath
 import Language.Preprocessor.Unlit
-import qualified Language.Haskell.Exts.Extension as HSE
 import qualified Data.Map.Strict as Map
 
 fakeSettings :: Settings
@@ -75,12 +74,7 @@
  ]
 
 enabledExtensions :: [Extension]
-enabledExtensions = [x | x <- [Cpp .. StarIsType], x `notElem` badExtensions]
--- 'Cpp' are the first and last cases of type 'Extension' in
--- 'libraries/ghc-boot-th/GHC/LanguageExtensions/Type.hs'. When we are
--- on a version of GHC that has MR
--- https://gitlab.haskell.org/ghc/ghc/merge_requests/826, we can
--- replace them with 'minBound' and 'maxBound' respectively.
+enabledExtensions = filter (`notElem` badExtensions) enumerateExtensions
 
 baseDynFlags :: DynFlags
 baseDynFlags = foldl' xopt_set
@@ -207,11 +201,16 @@
 isDotApp (OpApp _ _ (L _ op) _) = isDot op
 isDotApp _ = False
 
--- | A mapping from 'HSE.KnownExtension' values to their
--- 'GHC.LanguageExtensions.Type.Extension' equivalents.
-hseToGhcExtension :: Map.Map HSE.KnownExtension Extension
-hseToGhcExtension =
-  let ghcExts = Map.fromList [(show x, x) | x <- [Cpp .. StarIsType]]
-  in
-    Map.fromList [ (x, ext) | x <- [minBound .. maxBound]
-                 , Just ext <- [Map.lookup (show x) ghcExts] ]
+-- | All available GHC extensions
+enumerateExtensions :: [Extension]
+-- 'Cpp' are the first and last cases of type 'Extension' in
+-- 'libraries/ghc-boot-th/GHC/LanguageExtensions/Type.hs'. When we are
+-- on a version of GHC that has MR
+-- https://gitlab.haskell.org/ghc/ghc/merge_requests/826, we can
+-- replace them with 'minBound' and 'maxBound' respectively.
+enumerateExtensions = [Cpp .. StarIsType]
+
+-- | Parse a GHC extension
+readExtension :: String -> Maybe Extension
+readExtension x = Map.lookup x exts
+  where exts = Map.fromList [(show x, x) | x <- [Cpp .. StarIsType]]
diff --git a/src/HSE/All.hs b/src/HSE/All.hs
--- a/src/HSE/All.hs
+++ b/src/HSE/All.hs
@@ -239,8 +239,10 @@
    partitionEithers $ mapMaybe toEither exts
    where
      toEither ke = case ke of
-       EnableExtension e  -> Left  <$> Map.lookup e hseToGhcExtension
-       DisableExtension e -> Right <$> Map.lookup e hseToGhcExtension
+       EnableExtension e  -> Left  <$> readExtension (show e)
+       DisableExtension e -> Right <$> readExtension (show e)
+       UnknownExtension ('N':'o':e) -> Right <$> readExtension e
+       UnknownExtension e -> Left <$> readExtension e
 
 -- | Parse a Haskell module. Applies the C pre processor, and uses
 -- best-guess fixity resolution if there are ambiguities.  The