diff --git a/NanoID.cabal b/NanoID.cabal
--- a/NanoID.cabal
+++ b/NanoID.cabal
@@ -1,5 +1,5 @@
 name:                NanoID
-version:             2.0.0
+version:             2.1.0
 synopsis:            NanoID generator
 description:         Library and CLI tool for NanoID generation
 license:             BSD3
diff --git a/ReadMe.md b/ReadMe.md
--- a/ReadMe.md
+++ b/ReadMe.md
@@ -2,7 +2,7 @@
 
 ```
 [user@box ~] $ nanoid -h
-nanoid v2.0.0, (c) Michel Boucey 2021
+nanoid v2.1.0, (c) Michel Boucey 2021
 
 Usage: nanoid [-a|--alphabet ARG] [-l|--length ARG] [-q|--quantity ARG] 
               [-n|--newline]
diff --git a/app/Main.hs b/app/Main.hs
--- a/app/Main.hs
+++ b/app/Main.hs
@@ -13,8 +13,8 @@
 main :: IO ()
 main = do
   Options{..} <- execParser opts
-  if length < 1 
-    then putStrLn "Strange (nano)idea... less than one char" >> exitFailure
+  if length < 1 || quantity < 1
+    then putStrLn "Bad numeric input. See help (-h)." >> exitFailure
     else do
       let alphabet' = Alphabet { unAlphabet = C.pack alphabet }
       replicateM_ quantity $
diff --git a/app/Options.hs b/app/Options.hs
--- a/app/Options.hs
+++ b/app/Options.hs
@@ -16,7 +16,7 @@
 opts = info (options <**> helper)
   ( fullDesc
     <> progDesc "NanoID generator"
-    <> header "nanoid v1.0.0, (c) Michel Boucey 2021" )
+    <> header "nanoid v2.1.0, (c) Michel Boucey 2021" )
 
 options :: Parser Options
 options =
diff --git a/src/Data/NanoID.hs b/src/Data/NanoID.hs
--- a/src/Data/NanoID.hs
+++ b/src/Data/NanoID.hs
@@ -15,16 +15,19 @@
 nanoID = createSystemRandom >>= customNanoID defaultAlphabet Nothing
 
 customNanoID :: Alphabet -> Maybe Length -> GenIO-> IO (Either String NanoID)
-customNanoID a l g =
+customNanoID a l g = do
+  let l' = fromMaybe 21 l
+  when (l' < 1) (fail "Negative length for a NanoID")
   let ua = unAlphabet a
-      al = C.length ua in
-  pure . NanoID . C.pack <$> replicateM (fromMaybe 21 l) ((\r -> C.index ua (r-1)) <$> uniformR (1,al) g)
+      al = C.length ua
+  pure . NanoID . C.pack <$> replicateM l' ((\r -> C.index ua (r-1)) <$> uniformR (1,al) g)
 
 -- | The default 'Alphabet', made of URL-friendly symbols.
 defaultAlphabet :: Alphabet
 defaultAlphabet = Alphabet (C.pack "ABCDEFGHIJKLMNOPKRSTUVWXYZ_1234567890-abcdefghijklmnopqrstuvwxyz")
 
--- | Predefined 'Alphabet's borrowed from <https://github.com/CyberAP/nanoid-dictionary>
+-- * Predefined Alphabets borrowed from https://github.com/CyberAP/nanoid-dictionary
+
 numbers :: Alphabet
 numbers = Alphabet (C.pack "1234567890")