diff --git a/src/Monocle/Config.hs b/src/Monocle/Config.hs
index 10c66d99b..f0e058253 100644
--- a/src/Monocle/Config.hs
+++ b/src/Monocle/Config.hs
@@ -87,7 +87,7 @@ module Monocle.Config (
 
 import Data.Aeson qualified as Aeson
 import Data.ByteString qualified as BS
-import Data.Char (isLowerCase)
+import Data.Char (isLetter, isLowerCase)
 import Data.Either.Validation (Validation (Failure, Success))
 import Data.Map qualified as Map
 import Data.Text qualified as T (all, dropWhileEnd, isPrefixOf, null, replace, toUpper, uncons, unpack)
@@ -485,7 +485,7 @@ mkIndexName x = do
   let check name p = if p then Right () else Left name
   check "Is empty" $ not $ T.null x
   check "Is longer than 255 bytes" $ BS.length (T.encodeUtf8 x) < 256
-  check "Contains uppercase letter(s)" $ T.all isLowerCase x
+  check "Contains uppercase letter(s)" $ T.all (\x -> not (isLetter x) || isLowerCase x) x
   check "Includes [\\/*?\"<>| ,#:]" $ T.all (flip @_ @String notElem "\\/*?\"<>| ,#:") x
   check "Starts with [-_+.]" $ maybe False (flip @_ @String notElem "-_+." . fst) $ T.uncons x
   check "Is (.|..)" $ notElem x [".", ".."]