diff --git a/src/Servant/CLI/HasCLI.hs b/src/Servant/CLI/HasCLI.hs
index 6ca06b3..7400280 100644
--- a/src/Servant/CLI/HasCLI.hs
+++ b/src/Servant/CLI/HasCLI.hs
@@ -248,7 +248,7 @@ instance ( KnownSymbol sym
         addParam :: RequiredArgument mods a -> Request -> Request
         addParam = foldRequiredArgument (Proxy @mods) add (maybe id add)
         add :: a -> Request -> Request
-        add param = appendToQueryString (T.pack pName) (Just (toQueryParam param))
+        add param = appendToQueryString (T.pack pName) (Just $ T.encodeUtf8 $ toQueryParam param)
         opt :: Opt (RequiredArgument mods a)
         opt = Opt
           { optName = pName
diff --git a/stack.yaml b/stack.yaml
index 1a90f03..583ba6b 100644
--- a/stack.yaml
+++ b/stack.yaml
@@ -1 +1 @@
-resolver: lts-18.20
+resolver: lts-19.0