diff --git a/src/GEval/OptionsParser.hs b/src/GEval/OptionsParser.hs
index f7f351c..1e939b6 100644
--- a/src/GEval/OptionsParser.hs
+++ b/src/GEval/OptionsParser.hs
@@ -14,6 +14,8 @@ import Data.Maybe
 import System.IO
 import Data.String.Here
 
+import Data.Monoid ((<>))
+
 import GEval.Core
 import GEval.CreateChallenge
 
diff --git a/stack.yaml b/stack.yaml
index 9b27396..4bcdee5 100644
--- a/stack.yaml
+++ b/stack.yaml
@@ -2,4 +2,4 @@ flags: {}
 packages:
 - '.'
 extra-deps: [cond-0.4.1.1,murmur3-1.0.3]
-resolver: lts-6.26
+resolver: lts-9.5