diff --git a/src/Data/RomanNum.hs b/src/Data/RomanNum.hs
index 731da2e..f12b3fe 100644
--- a/src/Data/RomanNum.hs
+++ b/src/Data/RomanNum.hs
@@ -8,6 +8,7 @@ module Data.RomanNum (
     , validate
   ) where
 
+import Data.Char (toUpper)
 import Data.Map (Map, (!))
 import qualified Data.Map as Map (fromList)
 import Text.Read (readMaybe)
@@ -55,7 +56,7 @@ validate :: String -> Bool
 validate = either (\_ -> False) (\_ -> True) . parse
 
 readDigit :: Char -> Either String Int
-readDigit c = maybe errMsg (Right . (values !)) $ readMaybe [c]
+readDigit c = maybe errMsg (Right . (values !)) $ readMaybe [toUpper c]
   where
     errMsg = Left $ "Not a roman digit: «" ++ c:"»"