diff --git a/names-th.cabal b/names-th.cabal
--- a/names-th.cabal
+++ b/names-th.cabal
@@ -1,5 +1,5 @@
 name:                names-th
-version:             0.3.0.0
+version:             0.3.0.1
 synopsis:            Manipulate name strings for TH
 description:         This package includes functions to manipulate name string
                      and extra library functions for Template Haskell.
@@ -8,11 +8,13 @@
 license-file:        LICENSE
 author:              Kei Hibino
 maintainer:          ex8k.hibino@gmail.com
-copyright:           Copyright (c) 2013-2018 Kei Hibino, 2015 Shohei Murayama
+copyright:           Copyright (c) 2013-2020 Kei Hibino, 2015 Shohei Murayama
 category:            Development
 build-type:          Simple
 cabal-version:       >=1.10
-tested-with:           GHC == 8.4.1, GHC == 8.4.2, GHC == 8.4.3
+tested-with:           GHC == 8.8.1, GHC == 8.8.2
+                     , GHC == 8.6.1, GHC == 8.6.2, GHC == 8.6.3, GHC == 8.6.4, GHC == 8.6.5
+                     , GHC == 8.4.1, GHC == 8.4.2, GHC == 8.4.3, GHC == 8.4.4
                      , GHC == 8.2.1, GHC == 8.2.2
                      , GHC == 8.0.1, GHC == 8.0.2
                      , GHC == 7.10.1, GHC == 7.10.2, GHC == 7.10.3
@@ -22,13 +24,17 @@
 
 library
   exposed-modules:
-                     Language.Haskell.TH.Name.CamelCase
-                     Language.Haskell.TH.Lib.Extra
+                       Language.Haskell.TH.Name.CamelCase
+                       Language.Haskell.TH.Lib.Extra
   build-depends:       base <5, containers, template-haskell
-  hs-source-dirs:    src
-  ghc-options:       -Wall
+  hs-source-dirs:      src
+  ghc-options:         -Wall
+  if impl(ghc >= 8)
+    ghc-options:         -Wcompat
+  if impl(ghc >= 8) && impl(ghc < 8.8)
+    ghc-options:         -Wnoncanonical-monadfail-instances
 
-  default-language:     Haskell2010
+  default-language:    Haskell2010
 
 
 source-repository head
diff --git a/src/Language/Haskell/TH/Name/CamelCase.hs b/src/Language/Haskell/TH/Name/CamelCase.hs
--- a/src/Language/Haskell/TH/Name/CamelCase.hs
+++ b/src/Language/Haskell/TH/Name/CamelCase.hs
@@ -25,7 +25,7 @@
   toVarExp, toVarPat,
   ) where
 
-import Data.Char (toUpper, toLower)
+import Data.Char (toUpper, toLower, isLetter, isDigit)
 import Data.Set (Set, fromList, member)
 import Language.Haskell.TH
   (Name, mkName, TypeQ, conT, ExpQ, conE, varE, PatQ, varP)
@@ -38,6 +38,24 @@
 unCapitalize (c:cs) = toLower c : cs
 unCapitalize ""     = ""
 
+-- Adds a _ to the identifier which does not start with a letter or an
+-- underscore.
+letterStart :: String -> String
+letterStart (c:cs) | c == '_'   ||
+                     isLetter c    = c:cs
+                   | otherwise     = '_':c:cs
+letterStart ""                     = ""
+
+-- Only letters, digits, underscores and single quotes are allowed in an
+-- identifier.
+allowedChars :: String -> String
+allowedChars cs = map replaceUnallowed cs
+  where
+    replaceUnallowed c | isLetter c    ||
+                         isDigit  c    ||
+                         c `elem` "_'"    = c
+                       | otherwise        = '_'
+
 -- | rename the string that equals to reserved identifiers.
 rename :: String -> String
 rename cs | cs `member` reservedIds = cs ++ "_"
@@ -62,14 +80,16 @@
 
 -- | Make constructor name from 'String'.
 toConName :: String -> ConName
-toConName =  ConName . mkName . rename . capitalize
+toConName =  ConName . mkName . rename . capitalize .
+             allowedChars . letterStart
 
 -- | Type to wrap variable\'s 'Name'.
 newtype VarName = VarName { varName :: Name {- ^ Get wrapped 'Name' -} }
 
 -- | Make variable name from 'String'.
 toVarName :: String -> VarName
-toVarName =  VarName . mkName . rename . unCapitalize
+toVarName =  VarName . mkName . rename . unCapitalize .
+             allowedChars . letterStart
 
 -- | 'Char' set used from camel-cased names.
 nameChars :: String