diff --git a/lib/Echidna/Config.hs b/lib/Echidna/Config.hs
index f8d5777..3d761fe 100644
--- a/lib/Echidna/Config.hs
+++ b/lib/Echidna/Config.hs
@@ -13,8 +13,8 @@ import Control.Monad.State (StateT(..), runStateT)
 import Control.Monad.Trans (lift)
 import Data.Bool (bool)
 import Data.Aeson
+import Data.Aeson.KeyMap (keys)
 import Data.Has (Has(..))
-import Data.HashMap.Strict (keys)
 import Data.HashSet (fromList, insert, difference)
 import Data.Maybe (fromMaybe)
 import Data.Text (isPrefixOf)
@@ -23,11 +23,13 @@ import EVM.Types (w256)
 
 import qualified Control.Monad.Fail as M (MonadFail(..))
 import qualified Data.ByteString as BS
+import qualified Data.Aeson.Key as Key
+import qualified Data.HashSet as HS
 import qualified Data.List.NonEmpty as NE
 import qualified Data.Yaml as Y
 
 import Echidna.Test
-import Echidna.Types.Campaign 
+import Echidna.Types.Campaign
 import Echidna.Mutator.Corpus (defaultMutationConsts)
 import Echidna.Types.Config (EConfigWithUsage(..), EConfig(..))
 import Echidna.Types.Solidity
@@ -52,7 +54,7 @@ instance FromJSON EConfigWithUsage where
                   _        -> mempty
     (c, ks) <- runStateT (parser v') $ fromList []
     let found = fromList (keys v')
-    return $ EConfigWithUsage c (found `difference` ks) (ks `difference` found)
+    return $ EConfigWithUsage c (HS.map Key.toText $ found `difference` ks) (HS.map Key.toText $ ks `difference` found)
     -- this parser runs in StateT and comes equipped with the following
     -- equivalent unary operators:
     -- x .:? k (Parser) <==> x ..:? k (StateT)